Optimization opportunities (tracking issue)

There is a lot of room for improvements to the CPU usage of egui. Here are some things worth investigating:

• Try different hash algorithms for Id, FontCache and others.
• Precompute vertices for circles and rounded corners in epaint::Tessellator constructor (https://github.com/emilk/egui/pull/1547)
• Paint small filled circles using textured rects (by adding a few low-radius circles to the font atlas) (https://github.com/emilk/egui/pull/1616)
• Run egui with a proper profiler and see what pops up
  • Try https://github.com/mstange/perfrecord
• https://github.com/emilk/egui/issues/1098
• eframe/native: run App::update and the rendering in different threads
• Output a single veretex buffer and index buffer instead of Vec<ClippedPrimitive>
• emilk/egui#3862
• emilk/egui_plot#18,

Any PR should come with a benchmark.