Speed improvement of initial G-code preview:

1) Preallocating the vertex / index buffers to limit reallocation.
2) Inlining the pushing into the vertex / index buffers.
3) Running the vertex buffer generator on a limited number of threads
   as the generator does not scale well due to memory pressure.
   Not using all the threads leaves some of the threads to G-code
   generator.

src/slic3r/GUI/3DScene.cpp

@@ -1429,6 +1429,13 @@ static void thick_lines_to_geometry(
     double width_initial = 0.0;
     double bottom_z_initial = 0.0;
 
+    // Reserve for a smooth path. Likley the path will not be that smooth, but better than nothing.
+    // Allocated 1.5x more data than minimum.
+    // Number of indices, not triangles.
+    geometry.reserve_more_indices((lines.size() * 8 * 3) * 3 / 2);
+    // Number of vertices, not floats.
+    geometry.reserve_more_vertices(((lines.size() + 1) * 4) * 3 / 2);
+
     // loop once more in case of closed loops
     const size_t lines_end = closed ? (lines.size() + 1) : lines.size();
     for (size_t ii = 0; ii < lines_end; ++ii) {

src/slic3r/GUI/GLCanvas3D.cpp

@@ -6873,6 +6873,11 @@ void GLCanvas3D::_load_print_object_toolpaths(const PrintObject& print_object, c
         return volume;
     };
     const size_t    volumes_cnt_initial = m_volumes.volumes.size();
+    // Limit the number of threads as the code below does not scale well due to memory pressure.
+    // (most of the time is spent in malloc / free / memmove)
+    // Not using all the threads leaves some of the threads to G-code generator.
+    tbb::task_arena limited_arena(std::min(tbb::this_task_arena::max_concurrency(), 4));
+    limited_arena.execute([&ctxt, grain_size, &new_volume, is_selected_separate_extruder, this]{
     tbb::parallel_for(
         tbb::blocked_range<size_t>(0, ctxt.layers.size(), grain_size),
         [&ctxt, &new_volume, is_selected_separate_extruder, this](const tbb::blocked_range<size_t>& range) {
@@ -7047,6 +7052,7 @@ void GLCanvas3D::_load_print_object_toolpaths(const PrintObject& print_object, c
             vol->indexed_vertex_array.shrink_to_fit();
 #endif // ENABLE_LEGACY_OPENGL_REMOVAL
     });
+    }); // task arena
 
     BOOST_LOG_TRIVIAL(debug) << "Loading print object toolpaths in parallel - finalizing results" << m_volumes.log_memory_info() << log_memory_info();
     // Remove empty volumes from the newly added volumes.

src/slic3r/GUI/GLModel.cpp

@@ -94,22 +94,8 @@ void GLModel::Geometry::add_vertex(const Vec3f& position)
 void GLModel::Geometry::add_vertex(const Vec3f& position, const Vec2f& tex_coord)
 {
     assert(format.vertex_layout == EVertexLayout::P3T2);
-    vertices.emplace_back(position.x());
+    vertices.insert(vertices.end(), position.data(), position.data() + 3);
-    vertices.emplace_back(position.y());
+    vertices.insert(vertices.end(), tex_coord.data(), tex_coord.data() + 2);
-    vertices.emplace_back(position.z());
-    vertices.emplace_back(tex_coord.x());
-    vertices.emplace_back(tex_coord.y());
-}
-
-void GLModel::Geometry::add_vertex(const Vec3f& position, const Vec3f& normal)
-{
-    assert(format.vertex_layout == EVertexLayout::P3N3);
-    vertices.emplace_back(position.x());
-    vertices.emplace_back(position.y());
-    vertices.emplace_back(position.z());
-    vertices.emplace_back(normal.x());
-    vertices.emplace_back(normal.y());
-    vertices.emplace_back(normal.z());
 }
 
 void GLModel::Geometry::add_index(unsigned int id)
@@ -123,13 +109,6 @@ void GLModel::Geometry::add_line(unsigned int id1, unsigned int id2)
     indices.emplace_back(id2);
 }
 
-void GLModel::Geometry::add_triangle(unsigned int id1, unsigned int id2, unsigned int id3)
-{
-    indices.emplace_back(id1);
-    indices.emplace_back(id2);
-    indices.emplace_back(id3);
-}
-
 Vec2f GLModel::Geometry::extract_position_2(size_t id) const
 {
     const size_t p_stride = position_stride_floats(format);

src/slic3r/GUI/GLModel.hpp

@@ -4,6 +4,7 @@
 #include "libslic3r/Point.hpp"
 #include "libslic3r/BoundingBox.hpp"
 #include "libslic3r/Color.hpp"
+#include "libslic3r/Utils.hpp"
 #include <vector>
 #include <string>
 
@@ -85,13 +86,19 @@ namespace GUI {
             ColorRGBA color{ ColorRGBA::BLACK() };
 
             void reserve_vertices(size_t vertices_count) { vertices.reserve(vertices_count * vertex_stride_floats(format)); }
+            void reserve_more_vertices(size_t vertices_count) { vertices.reserve(next_highest_power_of_2(vertices.size() + vertices_count * vertex_stride_floats(format))); }
             void reserve_indices(size_t indices_count) { indices.reserve(indices_count); }
+            void reserve_more_indices(size_t indices_count) { indices.reserve(next_highest_power_of_2(indices.size() + indices_count)); }
 
             void add_vertex(const Vec2f& position);                          // EVertexLayout::P2
             void add_vertex(const Vec2f& position, const Vec2f& tex_coord);  // EVertexLayout::P2T2
             void add_vertex(const Vec3f& position);                          // EVertexLayout::P3
             void add_vertex(const Vec3f& position, const Vec2f& tex_coord);  // EVertexLayout::P3T2
-            void add_vertex(const Vec3f& position, const Vec3f& normal);     // EVertexLayout::P3N3
+            void add_vertex(const Vec3f& position, const Vec3f& normal) {    // EVertexLayout::P3N3
+                assert(format.vertex_layout == EVertexLayout::P3N3);
+                vertices.insert(vertices.end(), position.data(), position.data() + 3);
+                vertices.insert(vertices.end(), normal.data(), normal.data() + 3);
+            }
 
             void set_vertex(size_t id, const Vec3f& position, const Vec3f& normal); // EVertexLayout::P3N3
 
@@ -99,7 +106,11 @@ namespace GUI {
 
             void add_index(unsigned int id);
             void add_line(unsigned int id1, unsigned int id2);
-            void add_triangle(unsigned int id1, unsigned int id2, unsigned int id3);
+            void add_triangle(unsigned int id1, unsigned int id2, unsigned int id3){
+                indices.emplace_back(id1);
+                indices.emplace_back(id2);
+                indices.emplace_back(id3);
+            }
 
             Vec2f extract_position_2(size_t id) const;
             Vec3f extract_position_3(size_t id) const;