Slow processing during sketching

I am trying to create a model of a guitar fretboard. 21 frets, each .4mm wide, 1.5mm deep, 30mm long. The body is 6mm thick and 450mm long. While sketching the fret widths .2mm from fret center, the app just stays there thinking for about a minute or two, just drawing a .2mm line.I’m on an iPad Pro 12.9, latest model, iOS 15.5… what could be causing this?