I don't know if its something similar, but the eraser is very laggy regardless of anything.While I do like the brush tool in Opentoonz the most out of the other animation programs I've tried (when it doesn't lag), this is the only program where I've seen just normal vector lines slow down the program/brush.(if you are moving too fast or the program is running slow from the vector visibility) In otherwords, when the program can keep up, it registers the point the cursor is at and doesn't register your pen having gone anywhere between those two points (more clear in video if explanation not clear) It looks like the brush only registers when the program can keep up causing it to have a non curved appearance until you let go of the pen/cursor.

Onion Skin is basically unusable because the onion skin is apparently being treated as more vectors being shown, causing a major slowdown for anything that isn't bare bones simplistic.

If you hide columns/levels while you draw, (so that less/no vectors other than current drawing are visible) everything goes back to normal speed until the drawing you are making reaches a lot of vectors/line mileage.

Onion skin is also almost unusable because of this as explained below. It becomes very hard/completely impossible to draw comfortably/efficiently at a certain point.

The brush tool is fine when you are first starting off, but as more vector lines appear on screen (I've only tested vector), the brush starts slowing down more and more. I put a link at the bottom to an example video if you wish to see the issue visually. I hadn't seen any solutions in the google groups forum at all for this issue yet, so I thought I'd pass the issue down here.

Toon boom harmony brushes