well it's a bit hard to compare since mimalloc is a specific implementation
while arena is a general technique (which can have many different
implementations with different performance characteristics) but if i had to
guesstimate, in general a basic arena will always outperform mimalloc
allocations by 5-50x (totally random numbers, don't quote me on this) since a
suballocation in an arena is literally just bumping a pointer, which is
essentially free. but of course, arenas can have extra features which will
reduce allocation speeds by a lot, like managing freelists for reusing memory in
an arena, etc. just to get an idea of how many different implementations there
can be, there's a comparison of rust arena implementations here:
https://donsz.nl/blog/arenas/ (no performance numbers or anything, but just a
feature-wise comparison).

creating a comparison between mimalloc and arena allocators will always be
unfair (in favor of arenas) since arenas are not meant for general purpose
memory allocation and the arena can just choose not to implement the things that
mimalloc does (which make it slower)