I'm running X2a, build 351, 64 bit. Near as I can tell from downloads, this is latest.
When I do a Save, and in several other operations, X2 just goes unresponsive. Windows says its Not Responding, wait cursor diddles along for almost a minute or so. This happens during edit, too. 

I didn't use to have this problem, nearly as I recall. I running 64 bit Win 7. HP Pavilion 6620f, 16 GB RAM. AMD II X4 830 2.8 Ghz Processor. Tons of disk space.

Has anyone seen this behavior? It's not only disconcerting, it makes project editing a pain.

I haven't. Maybe you have a disk drive problem. Did you run scandisk on your disk drive? 

I've seen this behavior when saving projects. I have no idea why it does it since 8.5.3, X1, Ableton Live, Reason, and Sound Forge don't have this problem on the same machine. I've only had it on an edit once, and that actually crashed my computer. Why it happens...I have no idea.

i would focus directly on what plug ins you are using in your project,
remove them and try and save your project,did this help?

as much as it is a pain,you may need to go through one by one,plug ins and soft sythns.
after you have ruled this out,i would look exclusively at your hard drive,its age,its size and de-fragmention

let me know