Gotcha. I ask because, on my machine, kiUi takes 8-9ms every frame regardless of what I'm doing (e.g., typing, dragging windows, resizing things). If I'm trying to maintain 60 FPS with Emscripten, 8-9ms is a significant portion of my 16.7ms frame budget.
statico: you can load a different TTF with ImGui, it doesn't have to be monospaced and they can be smooth. The perf differences are due to designing the library with performances as a major pillar, lots of things are optimised. A visual redesign is planned but it'll take a bit of time to get it done to keep the perf up.
By comparison, a basic Emscripten libRocket demo takes 6-8ms (mostly due to Freetype rendering and its layout system, see http://forums.librocket.com/viewtopic.php?f=2&t=5223), and ImGui takes 1ms (probably due to its monospaced bitmap fonts and simple widgets, demo: http://floooh.github.io/oryol/ImGuiDemo.html).
Just some thoughts in case you're planning on adding complicated game logic every frame :)