That sounds like a bug, although changing fingerings is not a timely process, it is not guaranteed to happen immediately, so you may need to do that in a different way (although it ought to work doing it as you have, just be slow).
'Not timely' or 'slow' doesn't mean minutes, it just means that the new fingering might take a second or two to load as that is done at a lower priority than the sound processing and the lower priority stuff can be appreciably slower if your system is heavily loaded. Many actions in Belcanto have a pre-optimised, speedy executon than means they occur effectively instantaneously, which is very handy in performance. Changing fingerings is not one of those actions, it has to read the fingerings file in and process it, which takes a little while.
It would be helpful if you could file a bug using the bug reporter on this, it can help us track the problem down.
John