diff --git a/src/libslic3r/ClipperUtils.cpp b/src/libslic3r/ClipperUtils.cpp
index 86653598b0..0c36cc3f11 100644
--- a/src/libslic3r/ClipperUtils.cpp
+++ b/src/libslic3r/ClipperUtils.cpp
@@ -20,6 +20,9 @@
 #include "libslic3r/Surface.hpp"
 #include "libslic3r/libslic3r.h"
 
+#include <oneapi/tbb/blocked_range.h>
+#include <oneapi/tbb/parallel_reduce.h>
+
 // #define CLIPPER_UTILS_TIMING
 
 #ifdef CLIPPER_UTILS_TIMING
@@ -745,6 +748,8 @@ Slic3r::Polygons union_(const Slic3r::Polygons &subject, const ClipperLib::PolyF
     { return to_polygons(clipper_do<ClipperLib::Paths>(ClipperLib::ctUnion, ClipperUtils::PolygonsProvider(subject), ClipperUtils::EmptyPathsProvider(), fillType, ApplySafetyOffset::No)); }
 Slic3r::Polygons union_(const Slic3r::ExPolygons &subject)
     { return _clipper(ClipperLib::ctUnion, ClipperUtils::ExPolygonsProvider(subject), ClipperUtils::EmptyPathsProvider(), ApplySafetyOffset::No); }
+Slic3r::Polygons union_(const Slic3r::Polygons &subject, const Slic3r::Polygon &subject2)
+    { return _clipper(ClipperLib::ctUnion, ClipperUtils::PolygonsProvider(subject), ClipperUtils::SinglePathProvider(subject2.points), ApplySafetyOffset::No); }
 Slic3r::Polygons union_(const Slic3r::Polygons &subject, const Slic3r::Polygons &subject2)
     { return _clipper(ClipperLib::ctUnion, ClipperUtils::PolygonsProvider(subject), ClipperUtils::PolygonsProvider(subject2), ApplySafetyOffset::No); }
 Slic3r::Polygons union_(Slic3r::Polygons &&subject, const Slic3r::Polygons &subject2) { 
@@ -1012,6 +1017,21 @@ Polygons union_pt_chained_outside_in(const Polygons &subject)
     return retval;
 }
 
+Polygons union_parallel_reduce(const Polygons &subject)
+{
+    return tbb::parallel_reduce(
+        tbb::blocked_range<size_t>(0, subject.size()), Polygons(),
+        [&subject](tbb::blocked_range<size_t> range, Polygons partial_union) {
+            for (size_t subject_idx = range.begin(); subject_idx < range.end(); ++subject_idx) {
+                partial_union = union_(partial_union, subject[subject_idx]);
+            }
+            return partial_union;
+        },
+        [](const Polygons &a, const Polygons &b) {
+            return union_(a, b);
+        });
+}
+
 Polygons simplify_polygons(const Polygons &subject)
 {
     CLIPPER_UTILS_TIME_LIMIT_MILLIS(CLIPPER_UTILS_TIME_LIMIT_DEFAULT);
diff --git a/src/libslic3r/ClipperUtils.hpp b/src/libslic3r/ClipperUtils.hpp
index 00931e5268..6a01adef1f 100644
--- a/src/libslic3r/ClipperUtils.hpp
+++ b/src/libslic3r/ClipperUtils.hpp
@@ -520,6 +520,7 @@ inline Slic3r::Lines intersection_ln(const Slic3r::Line &subject, const Slic3r::
 Slic3r::Polygons union_(const Slic3r::Polygons &subject);
 Slic3r::Polygons union_(const Slic3r::ExPolygons &subject);
 Slic3r::Polygons union_(const Slic3r::Polygons &subject, const ClipperLib::PolyFillType fillType);
+Slic3r::Polygons union_(const Slic3r::Polygons &subject, const Slic3r::Polygon &subject2);
 Slic3r::Polygons union_(const Slic3r::Polygons &subject, const Slic3r::Polygons &subject2);
 Slic3r::Polygons union_(const Slic3r::Polygons &subject, const Slic3r::ExPolygon &subject2);
 // May be used to "heal" unusual models (3DLabPrints etc.) by providing fill_type (pftEvenOdd, pftNonZero, pftPositive, pftNegative).
@@ -538,6 +539,11 @@ ClipperLib::PolyTree union_pt(const Slic3r::ExPolygons &subject);
 
 Slic3r::Polygons union_pt_chained_outside_in(const Slic3r::Polygons &subject);
 
+// Perform union operation on Polygons using parallel reduction to merge Polygons one by one.
+// When many detailed Polygons overlap, performing union over all Polygons at once can be quite slow.
+// However, performing the union operation incrementally can be significantly faster in such cases.
+Slic3r::Polygons union_parallel_reduce(const Slic3r::Polygons &subject);
+
 ClipperLib::PolyNodes order_nodes(const ClipperLib::PolyNodes &nodes);
 
 // Implementing generalized loop (foreach) over a list of nodes which can be
diff --git a/src/libslic3r/TriangleMeshSlicer.cpp b/src/libslic3r/TriangleMeshSlicer.cpp
index 81d555aba6..d52d126e7f 100644
--- a/src/libslic3r/TriangleMeshSlicer.cpp
+++ b/src/libslic3r/TriangleMeshSlicer.cpp
@@ -2434,7 +2434,10 @@ Polygons project_mesh(
     std::vector<Polygons> top, bottom;
     std::vector<float>    zs { -1e10, 1e10 };
     slice_mesh_slabs(mesh, zs, trafo, &top, &bottom, throw_on_cancel);
-    return union_(top.front(), bottom.back());
+
+    // We typically perform a union operation on a lot of overlapping polygons, which can be slow in some cases.
+    // To address this, we use parallel reduction, which can be significantly faster in such cases.
+    return union_(union_parallel_reduce(top.front()), union_parallel_reduce(bottom.back()));
 }
 
 void cut_mesh(const indexed_triangle_set &mesh, float z, indexed_triangle_set *upper, indexed_triangle_set *lower, bool triangulate_caps)