I have noticed that, under normal usage, my laptop running KDE on Fedora 24 is fast and responsive. However, when system tray notifications display animations, the CPU usage of Xorg and plasmashell rise, with Xorg reaching 20% and plasmashell reaching 30%. My laptop has 4 CPU cores and this usage appears to correspond with two cores being completely occupied, bringing my laptop's performance down considerably. The effect ends immediately after the animated system tray icons stop animating, or are removed. The two that I have found cause this the most reliably are the Network Manager plasma widget and the Skype notification icon, which leads me to believe it is consistent across plasma widgets and icons registered with libnotify or similar.
*** This bug has been marked as a duplicate of bug 356479 ***
This bug is fixed in master since November. If you are indeed using the latest git master version, and still get this issue, please add a comment.
Sorry for the noise, it wasn't until I searched more thoroughly that I found the duplicates. After an update today, it appears to be resolved. Thanks!