diff --git a/t/cooling.t b/t/cooling.t
index a413a2b332..80819bdcb1 100644
--- a/t/cooling.t
+++ b/t/cooling.t
@@ -2,7 +2,7 @@ use Test::More;
 use strict;
 use warnings;
 
-plan tests => 12;
+plan tests => 13;
 
 BEGIN {
     use FindBin;
@@ -10,7 +10,7 @@ BEGIN {
     use local::lib "$FindBin::Bin/../local-lib";
 }
 
-use List::Util qw(first);
+use List::Util qw(none all);
 use Slic3r;
 use Slic3r::Test;
 
@@ -141,21 +141,33 @@ $config->set('disable_fan_first_layers', 0);
     $config->set('slowdown_below_layer_time', 10);
     $config->set('min_print_speed', 0);
     $config->set('start_gcode', '');
+    $config->set('first_layer_speed', '100%');
+    $config->set('external_perimeter_speed', 99);
     
     my $print = Slic3r::Test::init_print('20mm_cube', config => $config);
     my @layer_times = (0);  # in seconds
+    my %layer_external = ();  # z => 1
     Slic3r::GCode::Reader->new->parse(my $gcode = Slic3r::Test::gcode($print), sub {
         my ($self, $cmd, $args, $info) = @_;
         
         if ($cmd eq 'G1') {
             if ($info->{dist_Z}) {
                 push @layer_times, 0;
+                $layer_external{ $args->{Z} } = 0;
             }
             $layer_times[-1] += abs($info->{dist_XY} || $info->{dist_E} || $info->{dist_Z} || 0) / ($args->{F} // $self->F) * 60;
+            if ($args->{F} && $args->{F} == $config->external_perimeter_speed*60) {
+                $layer_external{ $self->Z }++;
+            }
         }
     });
-    my $all_below = !defined first { $_ > 0 && $_ < $config->slowdown_below_layer_time } @layer_times;
+    @layer_times = grep $_, @layer_times;
+    my $all_below = none { $_ < $config->slowdown_below_layer_time } @layer_times;
     ok $all_below, 'slowdown_below_layer_time is honored';
+    
+    # check that all layers have at least one unaltered external perimeter speed
+    my $external = all { $_ > 0 } values %layer_external;
+    ok $external, 'slowdown_below_layer_time does not alter external perimeters';
 }
 
 __END__
diff --git a/xs/src/libslic3r/GCode.cpp b/xs/src/libslic3r/GCode.cpp
index 2f17e91c39..01906526cf 100644
--- a/xs/src/libslic3r/GCode.cpp
+++ b/xs/src/libslic3r/GCode.cpp
@@ -201,7 +201,8 @@ Wipe::wipe(GCode &gcodegen, bool toolchange)
 
 GCode::GCode()
     : placeholder_parser(NULL), enable_loop_clipping(true), enable_cooling_markers(false), layer_count(0),
-        layer_index(-1), layer(NULL), first_layer(false), elapsed_time(0.0), volumetric_speed(0),
+        layer_index(-1), layer(NULL), first_layer(false), elapsed_time(0.0),
+        elapsed_time_bridges(0.0), elapsed_time_external(0.0), volumetric_speed(0),
         _last_pos_defined(false)
 {
 }
@@ -575,7 +576,9 @@ GCode::_extrude(ExtrusionPath path, std::string description, double speed)
     // extrude arc or line
     if (path.is_bridge() && this->enable_cooling_markers)
         gcode += ";_BRIDGE_FAN_START\n";
-    gcode += this->writer.set_speed(F, "", this->enable_cooling_markers ? ";_EXTRUDE_SET_SPEED" : "");
+    std::string comment = ";_EXTRUDE_SET_SPEED";
+    if (path.role == erExternalPerimeter) comment += ";_EXTERNAL_PERIMETER";
+    gcode += this->writer.set_speed(F, "", this->enable_cooling_markers ? comment : "");
     double path_length = 0;
     {
         std::string comment = this->config.gcode_comments ? description : "";
@@ -600,8 +603,12 @@ GCode::_extrude(ExtrusionPath path, std::string description, double speed)
     
     this->set_last_pos(path.last_point());
     
-    if (this->config.cooling)
-        this->elapsed_time += path_length / F * 60;
+    if (this->config.cooling) {
+        float t = path_length / F * 60;
+        this->elapsed_time += t;
+        if (path.is_bridge()) this->elapsed_time_bridges += t;
+        if (path.role == erExternalPerimeter) this->elapsed_time_external += t;
+    }
     
     return gcode;
 }
diff --git a/xs/src/libslic3r/GCode.hpp b/xs/src/libslic3r/GCode.hpp
index 8698eb1a6e..715af4d4c5 100644
--- a/xs/src/libslic3r/GCode.hpp
+++ b/xs/src/libslic3r/GCode.hpp
@@ -89,7 +89,7 @@ class GCode {
     // This value is not quite precise. First it only accouts for extrusion moves and travel moves,
     // it does not account for wipe, retract / unretract moves.
     // second it does not account for the velocity profiles of the printer.
-    float elapsed_time; // seconds
+    float elapsed_time, elapsed_time_bridges, elapsed_time_external; // seconds
     double volumetric_speed;
     
     GCode();
diff --git a/xs/src/libslic3r/GCode/CoolingBuffer.cpp b/xs/src/libslic3r/GCode/CoolingBuffer.cpp
index e976c9f465..786a9a8e40 100644
--- a/xs/src/libslic3r/GCode/CoolingBuffer.cpp
+++ b/xs/src/libslic3r/GCode/CoolingBuffer.cpp
@@ -19,8 +19,12 @@ CoolingBuffer::append(const std::string &gcode, std::string obj_id, size_t layer
     this->_gcode += gcode;
     // This is a very rough estimate of the print time, 
     // not taking into account the acceleration curves generated by the printer firmware.
-    this->_elapsed_time += this->_gcodegen->elapsed_time;
-    this->_gcodegen->elapsed_time = 0;
+    this->_elapsed_time          += this->_gcodegen->elapsed_time;
+    this->_elapsed_time_bridges  += this->_gcodegen->elapsed_time_bridges;
+    this->_elapsed_time_external += this->_gcodegen->elapsed_time_external;
+    this->_gcodegen->elapsed_time          = 0;
+    this->_gcodegen->elapsed_time_bridges  = 0;
+    this->_gcodegen->elapsed_time_external = 0;
     
     return out;
 }
@@ -55,32 +59,40 @@ std::string
 CoolingBuffer::flush()
 {
     GCode &gg = *this->_gcodegen;
+    std::string gcode = this->_gcode;
     
-    std::string gcode   = this->_gcode;
-    float elapsed       = this->_elapsed_time;
-    this->_gcode        = "";
-    this->_elapsed_time = 0;
-    this->_last_z.clear(); // reset the whole table otherwise we would compute overlapping times
-    
-    int fan_speed = gg.config.fan_always_on ? gg.config.min_fan_speed.value : 0;
-    
-    float speed_factor = 1.0;
+    int fan_speed           = gg.config.fan_always_on ? gg.config.min_fan_speed.value : 0;
+    float speed_factor      = 1.0;
+    bool slowdown_external  = true;
     
     if (gg.config.cooling) {
         #ifdef SLIC3R_DEBUG
-        printf("Layer %zu estimated printing time: %f seconds\n", this->_layer_id, elapsed);
+        printf("Layer %zu estimated printing time: %f seconds\n", this->_layer_id, this->_elapsed_time);
         #endif
         
-        if (elapsed < (float)gg.config.slowdown_below_layer_time) {
+        if (this->_elapsed_time < (float)gg.config.slowdown_below_layer_time) {
             // Layer time very short. Enable the fan to a full throttle and slow down the print
             // (stretch the layer print time to slowdown_below_layer_time).
             fan_speed = gg.config.max_fan_speed;
-            speed_factor = elapsed / (float)gg.config.slowdown_below_layer_time;
-        } else if (elapsed < (float)gg.config.fan_below_layer_time) {
+            
+            // We are not altering speed of bridges.
+            float time_to_stretch = this->_elapsed_time - this->_elapsed_time_bridges;
+            float target_time = (float)gg.config.slowdown_below_layer_time - this->_elapsed_time_bridges;
+            
+            // If we spend most of our time on external perimeters include them in the slowdown,
+            // otherwise only alter other extrusions.
+            if (this->_elapsed_time_external < time_to_stretch/2.) {
+                time_to_stretch -= this->_elapsed_time_external;
+                target_time     -= this->_elapsed_time_external;
+                slowdown_external = false;
+            }
+            
+            speed_factor = time_to_stretch / target_time;
+        } else if (this->_elapsed_time < (float)gg.config.fan_below_layer_time) {
             // Layer time quite short. Enable the fan proportionally according to the current layer time.
             fan_speed = gg.config.max_fan_speed
                 - (gg.config.max_fan_speed - gg.config.min_fan_speed)
-                * (elapsed - (float)gg.config.slowdown_below_layer_time)
+                * (this->_elapsed_time - (float)gg.config.slowdown_below_layer_time)
                 / (gg.config.fan_below_layer_time - gg.config.slowdown_below_layer_time);
         }
         
@@ -100,11 +112,12 @@ CoolingBuffer::flush()
                 if (boost::starts_with(line, "G1")
                     && boost::contains(line, ";_EXTRUDE_SET_SPEED")
                     && !boost::contains(line, ";_WIPE")
-                    && !bridge_fan_start) {
+                    && !bridge_fan_start
+                    && (slowdown_external || !boost::contains(line, ";_EXTERNAL_PERIMETER"))) {
                     apply_speed_factor(line, speed_factor, this->_min_print_speed);
                     boost::replace_first(line, ";_EXTRUDE_SET_SPEED", "");
                 }
-                bridge_fan_start = boost::contains(line, ";_BRIDGE_FAN_START");
+                bridge_fan_start = boost::starts_with(line, ";_BRIDGE_FAN_START");
                 new_gcode += line + '\n';
             }
             gcode = new_gcode;
@@ -125,6 +138,14 @@ CoolingBuffer::flush()
     }
     boost::replace_all(gcode, ";_WIPE", "");
     boost::replace_all(gcode, ";_EXTRUDE_SET_SPEED", "");
+    boost::replace_all(gcode, ";_EXTERNAL_PERIMETER", "");
+    
+    // Reset the buffer.
+    this->_elapsed_time          = 0;
+    this->_elapsed_time_bridges  = 0;
+    this->_elapsed_time_external = 0;
+    this->_gcode = "";
+    this->_last_z.clear(); // reset the whole table otherwise we would compute overlapping times
     
     return gcode;
 }
diff --git a/xs/src/libslic3r/GCode/CoolingBuffer.hpp b/xs/src/libslic3r/GCode/CoolingBuffer.hpp
index 01770a58d5..c290612ba4 100644
--- a/xs/src/libslic3r/GCode/CoolingBuffer.hpp
+++ b/xs/src/libslic3r/GCode/CoolingBuffer.hpp
@@ -17,7 +17,8 @@ and the print is modified to stretch over a minimum layer time.
 class CoolingBuffer {
     public:
     CoolingBuffer(GCode &gcodegen)
-        : _gcodegen(&gcodegen), _elapsed_time(0.), _layer_id(0)
+        : _gcodegen(&gcodegen), _elapsed_time(0.), _elapsed_time_bridges(0.),
+          _elapsed_time_external(0.), _layer_id(0)
     {
         this->_min_print_speed = this->_gcodegen->config.min_print_speed * 60;
     };
@@ -29,6 +30,8 @@ class CoolingBuffer {
     GCode*                      _gcodegen;
     std::string                 _gcode;
     float                       _elapsed_time;
+    float                       _elapsed_time_bridges;
+    float                       _elapsed_time_external;
     size_t                      _layer_id;
     std::map<std::string,float> _last_z;
     float                       _min_print_speed;