[RWP] Weird laggy performance issue

[Scott Chesworth]

Folks, I'm in a bit of a fix here, hoping someone can nudge me in the
right direction. I'm working on a mix that contains around 50 tracks
all told by the time you factor in buses and little snippets separated
for the sake of fake automation. There's a lot of VSTs in the project,
mainly amp sims. Minimal master bus processing, just the stock
multiband comp and a smidge of saturation happening there. Out of
nowhere, Reaper suddenly got really, really slow and laggy. As in, if
I hit play, stop, move to an item etc, it takes a good 5 seconds to
respond, and the tales of fx are all juddery and broken up. Here's
what I've looked at so far, but perhaps I missed something.

- I retraced my steps and removed the last few things added, no difference
- I've frozen pretty much every track I can. Aside from taking an age
to do that (presumably due to the same issue scuppering that process),
no difference.
I've rebuilt all peaks, just in case.
_ I've cranked the latency as high as it'll go in the control panel
for my interface, no difference.
- Tried switching to a different screen reader from my usual NVDA, no
difference.
- Loaded another similarly big project, which also behaves pretty
badly, so I'm guessing this is a setting of some description or some
sort of cache that needs flushing out.
- CPU load and RAM usage seem fine, lower than I would've expected
actually for a beast of a project like this

Any thoughts on what I might've missed? This is driving me barmy!

Scott