Incrementing or decrementing a shared counter is done with an atomic instruction, not with a locked critical section.

This has negligible overhead in most cases. For instance, if the shared counter is already in some cache memory the overhead is smaller than a normal non-atomic access to the main memory. The intrinsic overhead of an atomic instruction is typically about the same as that of a simple memory access to data that is stored in the L3 cache memory, e.g. of the order of 10 nanoseconds at most.

Moreover, many memory allocators use separate per-core memory heaps, so they avoid any accesses to shared memory that need atomic instructions or locking, except in the rare occasions when they interact with the operating system.