Hi David!
I’m using Windows 10.
The lag starts not too long after I start using Aseprite. 10 minutes maybe?
Changing settings in the options or restarting Aseprite doesn’t help either.
While this is happening, no matter if you use a tablet pen or mouse, the program seems to register the movement at a slow but constant rate, as you can see in the screenshot above.
Left is the result of me drawing circles slowly, on the right is when drawn at a faster rate.
P.S.)
Sorry for always complaining, the recent addon of Tilemap editor is doing wonders and I had a blast using it until I noticed this issue. 
(Though it needs multi-layer support for a single tilemap or rearranging the tilemap numbers)