Due to the quick fit algorithm, fragmentation is no longer an issue for memory allocators. Heap allocations are still a bit slower than stack allocations since

Due to the quick fit algorithm, fragmentation is no longer an issue for memory allocators. Heap allocations are still a bit slower than stack allocations since you need some way to release memory. Stack allocations are released at virtually zero cost (one assembly instruction). Hence sophisticated compilers perform escape analysis to convert heap allocations into cheaper stack allocations. But escape analysis, like all program analysis, is conservative and wont convert as many allocations as a human programmer could. However, in the grand scheme of things heap vs stack allocation is minuscule. Many other factors are much more important for performance.