I noticed that my computer at home could run applets fine but my work computer would stutter at 1 or 2 FPS. I finally found the discrepancy.
Go to Applications/Utilities/Java Preferences
Within the browser process
I switched to the other option (in their own process) thinking I’d get better performance and would avoid the occasional entire browser freeze from applets, but it turns out this totally borks everything, so don’t do it.
I can now play all the Java 4k games just fine on my Mac.