First you should read about pen smoothing and the different modes and what their settings do in Krita
When it’s turned off brush strokes should not lag but it kinda depends on what you mean with lag. There are different behaviors that could be referred to as lag.
If it still lags, similar issues in the past have been caused by simply using weak devices with ginormous brush and canvas sizes like here or here, or by giving Krita too few resources in the Settings.
What are your device specifications, how big is your canvas, your brush and what brush preset do you use when it’s happening?
Since you just wrote “Laptop” it could be the same issue since most laptops that are not for gaming or professional use are pretty weak to save battery.
I once had an issue with instant preview mode and had to turn it on/off but that was years ago.