debounceTime performance when having large amounts of updates

[voliva]

RxJS version:
6 and 7

Code to reproduce:
https://codesandbox.io/s/practical-poincare-gm0h6?file=/src/index.ts

It presents a stream with 100.000 updates, and two buttons to try two posibilities: use debounceTime from rxjs or use a custom (not finished) implementation that works with just one setTimeout

Expected behavior:
RxJS test takes a constant time, just like the custom implementation

Actual behavior:
RxJS test takes a linear time based on number of updates

Additional information:

There's a bit of discussion around debounce in #2834 and I can see that the implementation is not as straightforward as in the custom one, as there's an scheduler.

However, it should be posible to not cancel the task and re-create it on every message received, but use something similar to the custom implementation so that when the last task evaluates, it checks whether it needs to set up a new task or it can emit the last value straight away.

I think it's important to decrease the overhead of ignored messages, as one of the main points of debounceTime is to prevent processing quick succession of events.

I'm happy to contribute on this if you need some help.

[cartant]

Closed by #6049