Hey David thanks for your prompt response!
If I start with an empty project the lag is greatly reduced, still kinda there, but greatly reduced, then as soon as I start adding tracks, instruments, regions etc it gets worse and worse..
As a matter of fact I need to point out that LPX 10.4 also has a lag, but ONLY in the process of selecting a region, the blue elastic selection has no lag… And here too the lag gets worse and worse as the project builds up.