3.x: Performance improvements (tracking issue)

[akarnokd]

I assembled a fresh evaluation of RxJava 1, 2 and Reactor 3.2 performance under various usages and found the following deficiencies in RxJava 2.

• Flowable.empty() appears to be consuming a lot of memory.
• See if a non-trampolining Schedulers.from could improve async/pipeline performance.
• Check why Observable.blockingLast has 30% more overhead than its Flowable counterpart.
• Check why there is a lot more overhead with ReplayProcessor and ReplaySubject compared to v1.
• Check why there is a lot more overhead with UnicastProcessor and UnicastSubject compared to v1.
• Check why Observable.fromArray is somewhat slower with longer sources than Flowable.
• Check why Observable.fromIterable is slower with longer sources than Flowable.
• Optimize Observable.concatMap for scalar inner sources.
• Check why Flowable.flatMapIterable is drastically slower than Observable and Reactor's version in just/range/crossmap scenarios.

[ayudovin]

Hi, I'd like to check why you have a performance problem in this moment
if I had found the reason that Would I have opportunity to create a merge request with solution ?

[akarnokd]

@ayudovin Sure.

[akarnokd]

Flowable.empty() was simply a measurement anomaly due to class loading.