It sounds to me as though it is not known that your device is capable of storing thousands of projects without syncing them (those of us that don’t use Dropbox at all would have a fun time using Scrivener otherwise!). Think of the sync area more along the lines of being strictly a transfer mechanism. It’s where you put the stuff that actively changes on a regular basis from multiple contexts, with the idea toward making it more convenient to keep these contexts in parity. In the old days, this would be your floppy disk you use to carry changes around, not the hard drive where you do your actual work.
Most folks will not require archives of projects that never change to be stored in an area that is designed entirely around using the Internet to aggressively monitor and modify each device for changes. It would be like keeping all of your store’s inventory in the truck you use to pick up the morning doughnuts for the staff. The sync code is having to trawl through likely tens of thousands of folders and files, all to work with a small dozen or so that change from one day to the next. Even aside from the actual bug itself that is causing crashing, if one can reduce their sync times from minutes to seconds, why not?
Instead, drag old projects below the Dropbox bar in the project management list, and now they are “offline”, but readily accessible. If for some reason in the course of working through something, you find one of these older projects requires an important revision, it is an extremely simple matter of dragging it back above the Dropbox line to push those edits to all devices. You could at that point then store them “offline” on those devices if that’s the end of it, or leave it active for a while. Why not—it’s not a big deal at this point, since the sync folder is not overloaded at all times—a little extra probably won’t push things over the edge.
It’s not going to work for everyone, some people really do need everything synced, but I think most people out there only work in one or two projects at once. So for them, this isn’t really a “solution” to the actual bug, obviously, it’s more along the lines of general advice—but it becomes relevant to this thread when one gets the feeling that we’re saying you should do without having access to your work!