Thanks for all suggestions. Considering that Adobe Photography Plan cost is now almost 150 euro for one year and that for c.a. 400 euro I can buy an used laptop that is ~4x faster that my current one, I guess I wait a bit, gather some money and choose the second option. That will not only give me boost in Gimp (I hope big brushes 2.9 / 3.x with 16 bit will be usable and comfortable then), but also give nice boost to rest of the software that I use. Espermaschine - I also abandoned Lightroom when I abandoned PS. Second RAW converter that I own doesn't support local adjustments, so my current RAW conversion workflow is, that I am creating two or more bitmaps from the raw, with different exposure correction and sometimes other different settings (like white balance) too, load them all as the layers, add mask to the layers and unmask the parts of every layer that I need (for example to highlight some parts of the scene and send others into the background,retain detail in the parts of the image that were close to overexposed, etc.). I need those large brushes to paint layer masks - bigger brush gives much more uniform and nice looking effects when I want to dim larger parts of the image, corners, etc. Smaller brush leaves visible and ununiform patterns, especially on even surfaces.
As for now it seems that I can accept the average lag that I am getting with 2.8.22, but brush lag on 2.9.x (especially when in 16-bit mode) is way too big. Any chance that next Gimp versions (and stable 3.x) will have better performance?