I've been doing complex UI with a few different IMGUI libraries for years now and they can all lay out and rasterize complex scenes in under 2ms, even when writ

I've been doing complex UI with a few different IMGUI libraries for years now and they can all lay out and rasterize complex scenes in under 2ms, even when written in C# instead of performance-tuned C++. Modern hardware is just plain fast, in particular drop shadows and transparency are effectively free if you're drawing them using the GPU. (I say 'effectively' because if you opt out of transparency and antialiasing, you can do front-to-back rendering to skip drawing stuff entirely... but I don't know of many cases where people do that since it's not that much faster.)