This is so interesting.
I have horrible saving times right now.
Project is around 4 hours long. 400 tracks. A cpr file now is round 100mb big.
And I’m doing lots of global time copy, paste, delete stuff. Sounds strange that midi tracks can become damaged. That’s something I’d consider a bug 
But guess I need to go hunting through my MIDI tracks.
Also cleaning out the pool for unused audio didn’t help. Only reduced it with a couple of mb.