hi! im a newbie to this program and i use a mac air with 4gb ram and i5 and im trying to animate a bit not anything crazy. The size of the canvas is 1920x1080 and brush size is like 6-7 pixels. I seriously dont understand why it would lag. Do y'all have any suggestions?

Do you happen to have the stablizer turned on?

There's a bunch of other common culprits here: https://docs.krita.org/en/KritaFAQ.html#slow-brushes