Flexibility trades off with speed. As the power and flexibility of memory management increases, execution speed decreases.
The global memory manager (implemented by new() and delete()) is general purpose and consequently expensive.
Specialized memory managers can be more than an order of magnitude faster than the global one.
If you allocate mostly memory blocks of a fixed size, a specialized fixed-size memory manager will provide a significant performance boost.
A similar boost is available if you allocate mostly memory blocks that are confined to a single thread. A single-threaded memory manager will help by skipping over the concurrency issues that the global new() and delete() must handle.