Optimization opportunities (tracking issue)

[emilk]

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

• [x] Try different hash algorithms for Id, FontCache and others.
• [x] Precompute vertices for circles and rounded corners in epaint::Tessellator constructor (https://github.com/emilk/egui/pull/1547)
• [x] 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
• [ ] Optimize size of egui::Response (use bitfields?)
• [ ] eframe/native: run App::update and the rendering in different threads
• [ ] Output a single veretex buffer and index buffer instead of Vec<ClippedPrimitive>

Any PR should come with a benchmark.

[emilk]

I added integration with the puffin profiler in https://github.com/emilk/egui/pull/1483 to make it easier to a get a rough overview of app performance.

[YgorSouza]

This could be updated to link to #3862 and #1485, which have PRs in progress.