diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7cdd48eb32..121f314b1e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -115,13 +115,14 @@ function(cubos_common_target_options target)
     endif()
 
     if (EMSCRIPTEN)
-        set(LFLAGS "-sSTRICT -sFILESYSTEM=0 -sASSERTIONS=0 -sMIN_WEBGL_VERSION=2 -sMAX_WEBGL_VERSION=2")
+        set(LFLAGS "-sSTRICT -sFILESYSTEM=0 -sASSERTIONS=0 -sMIN_WEBGL_VERSION=2 -sMAX_WEBGL_VERSION=2 -pthread -sPTHREAD_POOL_SIZE=navigator.hardwareConcurrency")
 
         if (WITH_GLFW)
             set(LFLAGS "${LFLAGS} -sUSE_GLFW=3")
         endif ()
 
         set_target_properties(${target} PROPERTIES SUFFIX ".html" LINK_FLAGS ${LFLAGS})
+	    target_compile_options(${target} PUBLIC -pthread)
     endif ()
 endfunction()
 
diff --git a/core/include/cubos/core/gl/render_device.hpp b/core/include/cubos/core/gl/render_device.hpp
index 472babf4b3..7cb977017e 100644
--- a/core/include/cubos/core/gl/render_device.hpp
+++ b/core/include/cubos/core/gl/render_device.hpp
@@ -1103,6 +1103,11 @@ namespace cubos::core::gl
             /// @brief Unmaps the index buffer, updating it with data written to the mapped region.
             virtual void unmap() = 0;
 
+            /// @brief Fills the buffer with the given data.
+            /// @param data Pointer to data.
+            /// @param size Data size in bytes.
+            virtual void fill(const void* data, std::size_t size) = 0;
+
         protected:
             IndexBuffer() = default;
         };
@@ -1131,6 +1136,11 @@ namespace cubos::core::gl
             /// region.
             virtual void unmap() = 0;
 
+            // @brief Fills the buffer with the given data.
+            /// @param data Pointer to data.
+            /// @param size Data size in bytes.
+            virtual void fill(const void* data, std::size_t size) = 0;
+
         protected:
             VertexBuffer() = default;
         };
diff --git a/core/src/gl/ogl_render_device.cpp b/core/src/gl/ogl_render_device.cpp
index 5595725f6f..65a43615b2 100644
--- a/core/src/gl/ogl_render_device.cpp
+++ b/core/src/gl/ogl_render_device.cpp
@@ -833,6 +833,12 @@ class OGLIndexBuffer : public impl::IndexBuffer
         glUnmapBuffer(GL_ELEMENT_ARRAY_BUFFER);
     }
 
+    void fill(const void* data, std::size_t size) override
+    {
+        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, this->id);
+        glBufferSubData(GL_ELEMENT_ARRAY_BUFFER, 0, static_cast<GLsizeiptr>(size), data);
+    }
+
     std::shared_ptr<bool> destroyed;
 
     GLuint id;
@@ -878,6 +884,12 @@ class OGLVertexBuffer : public impl::VertexBuffer
         glUnmapBuffer(GL_ARRAY_BUFFER);
     }
 
+    void fill(const void* data, std::size_t size) override
+    {
+        glBindBuffer(GL_ARRAY_BUFFER, this->id);
+        glBufferSubData(GL_ARRAY_BUFFER, 0, static_cast<GLsizeiptr>(size), data);
+    }
+
     std::shared_ptr<bool> destroyed;
 
     GLuint id;
diff --git a/engine/src/imgui/imgui.cpp b/engine/src/imgui/imgui.cpp
index a1135e13a1..5a7175eae6 100644
--- a/engine/src/imgui/imgui.cpp
+++ b/engine/src/imgui/imgui.cpp
@@ -203,8 +203,6 @@ static void createDeviceObjects()
 
     // Setup shader pipeline.
     auto vs = rd.createShaderStage(gl::Stage::Vertex, R"glsl(
-#version 330 core
-
 layout (location = 0) in vec2 position;
 layout (location = 1) in vec2 uv;
 layout (location = 2) in vec4 color;
@@ -225,8 +223,6 @@ void main()
     )glsl");
 
     auto ps = rd.createShaderStage(gl::Stage::Pixel, R"glsl(
-#version 330 core
-
 uniform sampler2D tex;
 
 in vec2 fragUv;
@@ -289,6 +285,11 @@ void cubos::engine::imguiInitialize(io::Window window, float dpiScale)
         return;
     }
 
+// TODO: Fix this
+#ifdef __EMSCRIPTEN__
+    io.IniFilename = nullptr;
+#endif
+
     // Setup back-end capabilities flags
     auto* bd = IM_NEW(ImGuiData)();
     bd->window = window;
@@ -423,10 +424,9 @@ void cubos::engine::imguiEndFrame(const gl::Framebuffer& target)
     auto* drawData = ImGui::GetDrawData();
 
     // Upload projection matrix to constant buffer.
-    glm::mat4& proj = *(glm::mat4*)bd->cb->map();
-    proj = glm::ortho(drawData->DisplayPos.x, drawData->DisplayPos.x + drawData->DisplaySize.x,
-                      drawData->DisplayPos.y + drawData->DisplaySize.y, drawData->DisplayPos.y);
-    bd->cb->unmap();
+    auto proj = glm::ortho(drawData->DisplayPos.x, drawData->DisplayPos.x + drawData->DisplaySize.x,
+                           drawData->DisplayPos.y + drawData->DisplaySize.y, drawData->DisplayPos.y);
+    bd->cb->fill(&proj, sizeof(proj));
 
     // Set render state.
     setupRenderState(bd, target);
@@ -480,12 +480,8 @@ void cubos::engine::imguiEndFrame(const gl::Framebuffer& target)
         }
 
         // Upload vertex and index data into vertex buffer.
-        auto* vtxDst = (ImDrawVert*)bd->vb->map();
-        auto* idxDst = (ImDrawIdx*)bd->ib->map();
-        memcpy(vtxDst, cmdList->VtxBuffer.Data, static_cast<std::size_t>(cmdList->VtxBuffer.Size) * sizeof(ImDrawVert));
-        memcpy(idxDst, cmdList->IdxBuffer.Data, static_cast<std::size_t>(cmdList->IdxBuffer.Size) * sizeof(ImDrawIdx));
-        bd->vb->unmap();
-        bd->ib->unmap();
+        bd->vb->fill(cmdList->VtxBuffer.Data, static_cast<std::size_t>(cmdList->VtxBuffer.Size) * sizeof(ImDrawVert));
+        bd->ib->fill(cmdList->IdxBuffer.Data, static_cast<std::size_t>(cmdList->IdxBuffer.Size) * sizeof(ImDrawIdx));
 
         rd.setVertexArray(bd->va);
         rd.setIndexBuffer(bd->ib);