You may not have seen the post in the following link pai (as the thread it’s in is actually about over saturated colours when blending) but post 20 may be of interest to you.
Link:
https://forums.artrage.com/showthrea...224#post518224
In it AR user nekomata shares a few tips on making a custom brush.
One of the things in there that stood out to me was the size of the brush head image he used, only 250x250px.
Could be worth you experimenting with how much smaller you could go with yours and if it reduces brush lag.
I wonder if there is an optimum image size in terms of pixels for the Brush Designer to use?