Some of the state-of-the-art allocators close the performance gap pretty significantly, I was comparing to the classic Doug Lea style allocator. I imagine with something like your link the difference will be much less noticeable for allocation speed. However, not having an explicit free can lead to performance benefits, though it seems counter-intuitive. Imagine you are allocating a large number of small, short-lived objects. In the manual case, you have to explicitly call free() for each object as it goes out of scope. The free() call is rather fast for slab allocators, and free-ing everything runs in time O(dead_objects). A well-designed generational GC uses a copy collector for the eden generation, which essentially allocates a new slab and copies over all the live objects, and then frees the old slab. Copying objects is relatively fast for small objects, and this algorithm runs in time O(live_objects), which is much much smaller than the number of dead objects. Plus The copying step moves all the live objects together on the slab which reduces memory fragmentation.