ReplaySubject.Unsubscribe is very slow

ReplaySubject makes usage of ImmutableList to maintain a list of subscribers.

When calling ReplaySubject.Unsubscribe we will call ImmutableList.Remove, which is inefficient for various reasons.

It might be more efficient use a collection which uses GetHashCode.

Additionally, it should be noted, that most accesses to the ImmutableList uses lock(_gate). It might be possible to use a mutable list/hashset instead.