When transferring a bunch of files plasmashell uses a full cpu core and Xorg also uses a fair share. I'm suspecting the tiny animation - unchecking "Track file transfers and other jobs" in notification settings instantly removes the load. Disabling desktop effects makes no difference. Running KDE Neon 5.9.3 (QT 5.7.1) Intel graphics. kinfocenter says renderer is Mesa DRI Intel(R) HD Graphics 520 (Skylake GT2)
*** This bug has been marked as a duplicate of bug 378010 ***