Naw, it’s not just a Mavericks issue. It’s been happening on Macs since Lion and N4, and PCs at least as long; there have been multiple threads in regards to this problem.
I don’t know why I had one especially troublesome import from previous versions; let’s just call it a corrupt file. But… outside of that, the behavior in my case (and I work on both Macs and PCs, despite my sig), is pretty predictable, and occurs when:
-
The project is long-- has a lot of files.
-
There are lots and lots of edits in said files.
It’s not unusual for one of my projects to run 20 or 30 hours, and the editing’s pretty intense. A lot of offline processes, etc.
My workaround is to bounce the edited files and “Remove Unused Media” from the pool. I tend to trash the originals-- not good workflow for everyone, but it works for me 
but just removing them from the pool works as well, and if you save your project backups so that Autosave doesn’t zotz them over time you can have a comprehensive backup history.
Once the multitudinous edits and offline bounces are out of the pool, project saves and autosaves snappily again.
Would be great to get this (finally) fixed!
Chewy