Hello
KDENLIVE is lagging (providing that a ~2GB file has added on timeline)
System is: ryzen 1600x, amd, rx480, 16gb ram, windows 10 (clean; official)
1) while moving timeline by mouse scrolling or when ctrl+LMB dragging
2) During watch preview - when preview comes to the end of the showed timeline - lags begin (and timeline begin to move too; i think that it's the reason)
https://imgur.com/a/vESFFrS

I tried to repair it - I changed some options: 'environment'->processing threads' -> I set from 0 to 10
I didn't notice what any changes

Set the threat back to 1 because the playback is not really thread save. I see that you have chosen mixed track (V4, A3, V3, A3) and you set the preview to 540p. Save your project and make a copy. Load the copied project and choose "split audio tracks" and you get V4,V3,V2,V1/A1,A2,A3,A4. Does the playback is still that judder on the end of the timeline?