That's not what the quote means, really. They have best-in-class performance on both single- and multi-threaded workloads. It is just saying that the allocator

That's not what the quote means, really. They have best-in-class performance on both single- and multi-threaded workloads. It is just saying that the allocator supports a deferred_free callback, which is zero-overhead in the fast path and negligible overhead in the slow path.