Possible AsynchScheduler performance issue

[akaltar]

Hello! I'm using rxjs in a big project through redux-observable, and in certain conditions we spend over ~5s in pure rxjs code without executing any of our own code.

I believe I might have found the root cause for spending so much time "spinning wheels" in rxjs, and I might be able to come up with an optimization that could make things better, but I want to make sure this aligns with the way development is going.

First, here is a repo with a reproduction of the performance issue: https://github.com/akaltar/rxjs-perf-repro

I think the issue might be in the way the looping is done over the actions in AsyncScheduler.ts:flush

• the way an action removes itself from the parent schedulers.

I'm not very familiar with how rxjs works, but I think replacing the loop with a simple for-loop, and adding a fast-path to removing the action from the parent actions array could speed things up a lot wihtout breaking things.

Maybe by not trying to remove this from the actions array at all, since at that point in flush it was already removed by shift, in the for-loop scenario, we could maybe just clean up the actions by assigning a fresh empty array after executing. Should be a non-breaking change and gain quite a bit of performance
I think further gains could be had if actions were stored in a Map, but that would be a much bigger change. (A bit like #7234 just for AsyncScheduler)

In either case I'm happy to put together a PR if you don't see any fundamental issues with these suggestions

[gyhyfj]

I encountered with another performance issue when I was coding a script for crawling sth.
When running that script, it didn't cost much memo but always froze later without any error.
Then I replace from + concatMap API with normal for loop, it worked well again and much faster than before ( down from 20s to 5s per loop)