upcoming_windows is slow

[Mark2000]

Is your feature request related to a problem? Please describe.
The upcoming_windows, next_windows, and next_targets functions are slow, especially when called with a large number of targets.

Describe the solution you'd like
The API should probably stay the same; however, targets should be stored in a different data structure and perhaps accessed through different methods that don't require the construction of large dicts.

Describe alternatives you've considered
Caching may also be a viable option, though this introduces certain risks. In the case I'm currently looking at, it would lead to 4x fewer calls.