Here is a small demo that renders a lot of ImGui windows in OpenGL and Vulkan.
Results and + downloads links in the article:
https://www.geeks3d.com/hacklab/20180620/lot-of-imgui-windows-opengl-vs-vulkan/
(https://www.geeks3d.com/public/jegx/2018q2/geexlab-lot-of-imgui-windows-demo-opengl-02.jpg)