Hello there, I've been trying out the third beta of the 3.1 release on my 2015 13" MacBook Pro. I've noticed when I make fast brush strokes (or any speed that isn't slow) that, instead of proper circles, the lines are very angular and sharp no matter which brush I use and there appears to be a frame-rate drop when I do so. The frame-rate drop is only on the canvas, even when just hovering over it. I tried changing Instant Preview ON/OFF and changing the display settings a bit but it stays the same. I checked the same version on a Windows machine and it works fine, so it appears to be related to my MacBook Pro.
It seems it was my display resolution that was the cause. Sorry about this.
Ah -- thanks for checking back with us!