Now there are huge lags all of a sudden

I have a huge project I've been working on all year. It's now about 27 minutes long, with about 30 tracks. All year Sonar's been a real champ, never having performance issues with the size of the project, and I've been pretty delighted with that all along.
 
The other day I finally moved my project to a different folder, so that it would sync with Dropbox. The local sync folder is on the same physical HD as before. But now there are big lags in performance. When I move the now time from a marker near the beginning to a marker near the end, for example, it takes 4-5 seconds to catch up. Playback and recording still seems fine, but as this issue has suddenly come up only a couple days ago, I don't really have enough tests to say for sure it won't lag in other areas too. I'm trying to nip this in the bud.
 
To be clear, there have been NO such issues until a couple days ago after I moved the project to the different folder. So it's not like this lag has been growing and growing or anything. It's gone straight from no lag at all, to this 4-5 second thing.
 
Ideas?