I cannot use kirita due to its lagging. Every aspect, the brush and undo and redo functionality lags at least 5 seconds. If you could please tell me how to fix this issue, it would be most appreciated. I am new to Kirita and don't know how to use it correctly yet.
I'm sorry, but this is a bug tracker, not a user support forum, so this is not a valid bug report. Besides, without knowledge of your system, it's impossible to tell why Krita would be slow on that system. Check https://docs.krita.org/KritaFAQ#Krita_is_slow for some hints, and https://phabricator.kde.org/T7199 for more in depth information.
I just told you what the bug was, you asshole.
No, you did not report a bug. You reported a generic complaint, one that 100% sure is specific for your system, about which you didn't tell us anything. Without any information, we cannot help you, and now you've made it extremely unlikely that we'll ever want to help you.