Andrei Alexandrescu had a great talk[0] on allocators and this convinced me that if you really care about allocator performance[1] what you need is a tunable and composable system. The whole idea of how and when (runtime/compile-time) to choose the appropriate allocator is also interesting.

[0] https://www.youtube.com/watch?v=LIb3L4vKZ7U

[1] I suppose there's a digression to be had here about allocation being trivial and RAII-style deallocation not really being deterministic, but...