Narrowing this down:

• It appears that IF a blur filter is applied to an image, ON the image itself, using the pop-up window, and you attempt to paint on the image after the fact, GIMP will continue to render an updated blur on the image, with the paint appearing BEHIND the blur... thus the CPU lag.
• If you however undo the blur, re-select the area, and choose to Repeat the filter, then paint on that, the brush will paint OVER the filter (instead of behind), and the filter will not be re-rendering the preview output.

I cannot tell if this is intentional or not.

See video

@GIMP

#Gimp #Gimp3

it's a known limitation that we hope to fix soon, but hat requires more internal work. Thanks for describing it!