Well, that dose sound a bit odd…
Which Mac OS version are you on?
ARV’s current version is 7.1.4* and has been for quite some time now
My first recourse in these cases is to shutdown the system, have a cup of tea, and then restart it again. (The tea is optional!)
It’s old school I know but a lot of times it works to fix odd happenings!

If it still persists then…
Like you I’ve never had any issues with Multithreading and have always had it enabled in ARV Prefs.
Whether you having an M1 chip is relevant here I couldn’t say, as my system is still Intel based.
One possibility is a recent system update may have upset something?
Is your system set to auto install OS updates*?
A tablet driver update could also be a possible cause if you have installed a new one recently?

Which brushes seem slow now? All or just some?
The Oil brush is a good one to try as it’s computational load is not that high even at larger sizes. Some of the Custom Brushes on the other hand and the Watercolour tool at large sizes can have quite a high computational load and can lag.
But there are many variables, not least of which is Canvas size relative to Brush size. A big brush on a big canvas will be slower.

Regarding Key Commands, have you looked to see if they are still as you last set them?
If you have saved them regularly do you have a previous set you could load and test? (But save the currently loaded one first just in case!)

*For both OS and the App Store I have my system set to just advise me when updates are available, just so I can keep track of when an update was applied and if it coincide with any subsequent odd App or the System behavior.