Allocation is perceived as slow because it is. Getting memory from the OS is somewhat expensive because a page of memory needs to be allocated and stored off. Getting memory from traditional allocators is expensive because freespace needs to be tracked. When you say "I need 5 bytes" the allocator needs to find 5 free bytes to give back to you.
Bump allocators are fast because the operation of "I need 5 bytes" is incrementing the allocation pointer forward by 5 bytes and maybe doing a new page allocation if that's exhausted.
GC allocators are fast because they are generally bump allocators! The only difference is that when exhaustion happens the GC says "I need to run a GC".
Traditional allocators are a bit slower because they are typically something like an arena with skiplists used to find free space. When you free up memory, that skiplist needs to be updated.
But further, unlike bump and GC allocators another fault of traditional allocators is they have a tendency to scatter memory which has a negative impact on CPU cache performance. With the assumption that related memory tends to be allocated at the same time, GCs and bump allocators will colocate memory. But, because of the skiplist, traditional allocators will scattershot allocations to avoid free memory fragmentation.
All this said, for most apps this doesn't matter a whole lot. However, if you are doing a CPU/memory intense operation then this is stuff to know.