Release day: February 5th, 2019
Update 5.0.8 fixes the lag while writing.
We'll release another update soon that will also fix remaining problems with the eraser and scrolling performance.
We found out that most of the slow and lagging writing performance was related to a memory issue with the lasso tool. After using the lasso tool for multiple times, the resources required for writing weren't "freed" up by the system yet and so the pen tool didn't properly work. Quitting and reopening the app was freeing up these resources.
Since this problem was closely related to the lasso tool usage, not everyone was experiencing problems.