While OP might've phrased it more politely, he's got a point - allocators that aren't designed for multithreaded use are ultimately oversimplified toy projects.

It's really not that much of a challenge to knock together a (slab + heap + free list) allocator that will perform really well single-threadedly. However it will be nearly impossible to adapt it to the multithreaded context. It is a considerably more complex task and the end result will end up looking like a rocket ship compared to a simpleton that even the best single-threaded allocator will look like.

Doesn't the benchmark also just allocate and then immediately free? I think I could specialise for that particular use pattern and make it very fast just for the benchmarks. Will that not work for some reason? A good benchmark might be a replayed set of allocate and free operations from a large real application.