In OP's exact construction, the choice of preposition is effectively arbitrary. It can validly be "to", "of", or "for" - all of which are used by competent speakers.
That choice is not affected by whether there's one or more potential optimisations, nor does it affect the inherent ambiguity over whether those optimisations are mutually exclusive alternatives or not.
With the phrasing as given, I personally would make no assumption regarding that ambiguity. It's inherent in many constructions referring to [more than one] different way[s] to improve [something], whatever the precise wording. You need completely different phrasing to distinguish...
Either or both of two optimisations could be applied to this subprocess.
...from...
This subprocess can be optimised in one of two different ways.
FWIW, here are some usage figures from Google Books...
optimizations of the (14,200); for the (5020); to the (3840).
optimisations of the (1,100); for the (194); to the (156).