Steps below are reproducible at least with the Icons and Details views. Reproducible: Always Steps to Reproduce: 1. Copy a huge file from e.g. a FTP server or from a kdeconnect-enabled phone to Dolphin. 2. While Dolphin is busy copying the file above, start dragging another file Foo to some folder Bar. Actual Results: Lag: the icon of Foo does not quickly follows the mouse over the destination Bar. Expected Results: No lag: the icon of Foo follows the mouse as expected, over the destination Bar.
I am getting a similar behaviour. I have been seeing that for at least a few months. It could be that it has always been there since I installed Plasma 5 in July. The main difference with my experience is that even when dolphin is not doing anything drag and drop is already kind of "slow". The icon may typically lag about 1 second after the mouse pointer. This does not always happen with a quick drag and drop but moving the icon around a bit while dragging is sure to show that lag at some point. When there are some file operations in the background, then the lag may increase to 10 seconds or more. Apart from that, I did not notice anything really unusual. Only processes in the background using some significant CPU are plasmashell and kwin_x11 (both using up to 10% CPU according to "top", which seems high but might be normal). Xorg is usually at 5 to 10% CPU. I am running ArchLinux (up to date) with an NVIDIA GPU (proprietary driver).
Kubuntu 16.04 (Beta and Release), Nvidia proprietary: Same here since Plasma 5: When first copying files over the network drag and drop works fast. When starting to drag a second time (while the first copying is still in progress) the folder-icon follows the mouse like ist is connected with a "bubblegum" - ist means, the icon reaches the mouse about 3 seconds later.
I don't know about Kubuntu 16.04, but at least for me on ARCH, the bug seems to be gone with a recent update. I am now using KDE applications 16.04 but the bug apparently disappeared with a previous update (not sure which one exactly) a few weeks ago.
*** This bug has been marked as a duplicate of bug 352019 ***