hmmm.... how d'you know local arrays are accessed slower than global arrays?
My guess: because a global variable initialized in the start of the mission is already in memory when your script starts but a local variable needs to be initialized first in each script (placed into memory). And additionally, the speed to access array contents
after the variable has been placed into memory
can also be different for local and global but this can't be said without testing it and it's not what matters here.
It's a guess only from my part. This is the kind of things which can be said for sure only through testing.
I would say, make it global but remember to use your ofpec tag in the variable name.
THobson has a good point there to avoid global variables as much as possible. It's a normal programming way to give the variable a scope it needs, but not any more. In OFP we have to make exceptions sometimes (your global variable in OFP can be accessed from other scripts which don't actually need to access it at all).