Couldn't you speed up loops without annotations by compiling two versions: one that uses longs and one that uses bignums. You start in the loop that uses longs

Couldn't you speed up loops without annotations by compiling two versions: one that uses longs and one that uses bignums. You start in the loop that uses longs and before an operation that can overflow you check whether it overflows and if so you jump to the loop that uses bignums. This way you only suffer a few extra checks which is a lot cheaper than boxed numbers.