There are several levels at play here. malloc maintains a shared data structure that is used across cpus and must lock for access to that data structure. There

There are several levels at play here. malloc maintains a shared data structure that is used across cpus and must lock for access to that data structure. There are optimizations there being used with some alternative allocators such as per-cpu or per-thread pool but these only reduce the rate of locks taken. In addition after you have that virtual memory in hand you need to have physical memory behind it which entails a page fault which switches into the kernel to get you a physical page, this happens per-page. There is also the effect of physical memory layout on performance, if your memory is perfectly contiguous and your data is laid-out properly you can reap benefits from that compared to the quite likely fragmented nature of data in a heap-allocated setting. This obviously means that the applications needs to manage its memory allocations on its own but when you really care about performance these become important concepts and I too tend to write my libraries for such reuse and avoid memory allocations inside them where possible. malloc is a general purpose tool and as such needs to cater to many different environments, including multi-threaded programs so it has to lock its access to the shared data structures it holds to main