Serialization ensures mutual exclusion when two or more threads attempt to simultaneously allocate or free blocks from the same heap. There is a small performance cost, but it must be used when multiple threads allocate and free memory from the same heap.
A critical section is always used to serialize access to an individual heap. There is a critical section per heap to protect access to each heap.
An attempt to grab a critical section that is not owned is a fast path operation that incurs little overhead. This is similar to using the HEAP_NO_SERIALIZE flag if only one thread was ever accessing a particular heap.
If there is contention for the critical section, and therefore the heap, a new thread request to allocate heap space will serialize.