I have precisely the same issue, also with a shared calendar although my users file is 14MB - still not that big.
They had no problems with it when they were running version 5.x (yes I meant 5.x), it worked fine for them in that version but now they're using v 14.6 they have really slooowwwww speeds.
Over 65 seconds to save a new appointment!
I've uninstalled and reinstalled, deleted and recreated index files etc. but to no avail. Users who have an empty calendar have no issues saving new appointments.
I too think I'll have to go down the support ticket route, but doesn't sound like I'll get a resolution soon.
Poor users are pulling their hair out and I can understand why.