This was a problem for me too, I fixed it by
Exporting my anime files (look around in 3dots settings for it) and deleting all drawings other than the ones your working on. It will be MUCH better. Sadly if you want to draw on older drawings you need to export your current drawings and import the old ones. A bit of a pain but much less of a pain then lag.

I should note I'm on android, so I may or may not work on ios.

For paintbucket zoom all the way in to the middle of where you want to fill and click in the middle, the closer you are to something the more likely it will bug out

By color glitches do you mean the color tab shifting from where it should be? If that's the case the activation area is still the same, it's just visual so click where the colors would be without the shifting.

One final thing for lag, go into app settings on your device and clear cache, the app uses way to much and it causes lag. DO NOT CLEAR DATA

Let me know if it fixes it.