Tracking issue for performance improvements

I'd like to make a series of performance improvements to expert and thought it'd be good to get a list of major pain points/ issues

- [ ] Completion: This should be near instant (<200ms) even on slow machines and larger projects currently on my slow laptop it's(300-600ms)

- [ ] startup time:
- [ ] a lot of time is spent applying namespace when rebuilding the engine (it rebuilds every start for me, not sure if it should)
- [ ] After startup, to get the first completion, the first call to Modulestore.build() in elixir sense takes around 20s subsequent calls are much more reasonable.

I haven't noticed anything else being frustratingly slow. But my usage is limited and sometimes it's hard to tell if a feature is broken or just extraordinarily slow (like go-to def)