Only if the bottleneck is disk IO. Before going through a lot of effort to move OH to a tmpfs, maybe spend a little time proving that is in fact the source of your problem.
I’d be very surprised this is your problem. Once Rules are loaded into memory at OH start time they remain in memory. Disk IO can’t cause the lag. Moving to a tmpfs will improve how quickly those Rules can be loaded in the first place, not in how they run.
In all likelihood you have more than five Rules trying to run at the same time. See (OH 1.x and OH 2.x Rules DSL only] Why have my Rules stopped running? Why Thread::sleep is a bad idea.
There is no list. You can skip userdata/tmp and userdata/cache, but if you do OH will have to download and reinstall all the add-ons every time you reboot.