When moving large numbers of files from a folder to another folder using Dolphin, the task bar often becomes unresponsive. After a while, a message pops up on the system tray, saying that all transfers completed, and the problem goes away.
It seems that this takes much longer than the actual file transfer operation.
This is probably somehow related to the interaction between Dolphin and Plasmashell. Such problems do not occur if Thunar is used. Dolphin version is 16.12.3.
Today I noticed this problem on Arch Linux (plasma 5.11.5, dolphin 17.12) and neon dev unstable.
open dolphin and cut several small files (~150 jpg files smaller than 1 MiB, for example)
press F10 and create a new folder
right click on your just created folder, select "paste x items"
Result: panel freezes during some seconds. It immediately unfreezes when notification about finished file operation appears.
*** This bug has been marked as a duplicate of bug 358231 ***