I'm not sure it's a bug. Selecting color in Advanced Color Selector in the regular releases (including Krita 4.2.7) is a bit laggy, and it's smooth in the current nightly build. (git 835ca2e, and couple of past nightly builds before that.)

Just wanted to share.

That's...interesting.
I can confirm it is smoother, although it has never been laggy to a point where I really noticed it, only when I drag the handles in wild circles I really see the difference.

But I don't really have an idea what the reason might be. The advanced selector code itself has not really changed in ages apart from gamut masking.
And since my changes to the internal color selector (the one for the various color dialog windows) have been backported to 4.2.7, that's not the cause either, there must be some other slow listener to the color update signals (which the advanced selector probably could emit a bit less excessively too...)

I'm not sure it's relevant, but :
One more thing I've noticed is that it's slower when the krita window size is big. The smaller the krita window is, the faster it is.
I didn't check if it's the case in both nightly builds and the regular releases.

* I'm talking about selecting color in the square/triangle in the middle.