Not an expert but I don't agree. You're talking about bump allocators I assume, a nursery area. Do much of that and the TLB gets used more, which can get expensive.

If it's not bump-allocating then you have to traverse data structures to find a suitable block of free mem. At the other end, each alloc() must get a free() and reclaiming and unifying memory to prevent fragmentation isn't cheap I guess.