From this message (posted on the cassandra user ML) we can see that the tokenmap build process took 19 seconds which is too slow even though there are 100+ hosts with 256 vnodes each.
This affects the cluster startup time.
At the very least we should remove the duplicate code here https://github.com/datastax/csharp-driver/blob/43037b9c62d430e397aadf38499128b382552656/src/Cassandra/TokenMap.cs#L152-L164
(UpdateKeyspace has the exact same lines of code pretty much)
Also of interest here is the whole Java driver epic .