Resurrecting this thread to report that I’m having exactly the same issue, but with all of the SSL plugins I have: Bus Comp 2, Channel Strip 2, FlexVerb, Vocalstrip, X-EQ2, X-Saturator v6 and X-ValveComp v6.
It seems to be something to do with how SSL are rendering their GUIs. When the SSL GUI is open, then no problem. But if you close the GUI, then renoise slows down drastically - especially the keyboard input. It becomes so laggy with so much key repeat (i.e. if you hold down a key and then let go, it takes some time for all the key presses to clear), renoise’s interface becomes basically unusable until you remove the SSL plugin.
I’m on a Windows 10 machine running a Core i7-7700 CPU @ 3.60GHz with 16GB RAM. All other renoise settings are the same as @Garf above.
I mean, the workaround is to just not use SSL plugins at all… but this is all very strange. Never experienced anything like this with Renoise and any other plugins.