This is reported here: https://bugs.kde.org/show_bug.cgi?id=408133

I tested the infinite undo history with undo set to 20:

krita-nightly-x64-v4.2.0-HDR-298-g8f8660a2ea: works as expected
krita-nightly-x64-v4.2.0-HDR-307-g96b04eda2b: works as expected
krita-nightly-x64-v4.2.0-HDR-312-g0bb8b52570: works as expected
krita-nightly-x64-v4.2.0-HDR-324-g81407392c7: infinite undo stack
krita-nightly-x64-v4.2.0-HDR-328-g0a552f7b72: infinite undo stack
krita-nightly-x64-v4.2.0-HDR-331-g199cd9f521: infinite undo stack.

Not sure if this is related, but it falls within the builds I originally noticed this lag, although I thought 307 and 312 did not have this lag.

Hello. I also have a GT-221 Pro with Windows 10. Using the US comic book template I only have the lag issue when the brush is set to stabilizer. But that's a setting I don't really use. On the weighted setting there doesn't appear to be any lag issues on basic brush types. My graphics card is a GTX 1080 running on a i7-8086K.