It doesn't actually have "large memory allocations" due to immutable data structures. This is a meme that isn't true. Immutable data structures, especially at small scale, do not have huge performance penalties. You don't copy the entire structure over and over...you copy the O(log n) spine.
Haskell's GC is also fast when you are mostly generating garbage, which is inherently true for web server handlers.
Haskell's GC is also fast when you are mostly generating garbage, which is inherently true for web server handlers.