Thought experiment:-
- The icons appearing in Dolphin reside in a subfolder of
/usr/share/icons
and the specific subfolder is dependent upon which theme is used.
- The permissions of all icon files (and directories) must be
755, else they will not display – I think this is correct.
So, logically… If permissions other than 755 are applied to selected icons in that theme they will not display in Dolphin and the resources previously used will be freed.
Therefore, if the dolphin is made of wood, it must be a duck…
The 
Test!
I’m in another unspecified OS currently and can’t test this, so I’ll just wait, and doodle a while till someone chimes in with their results.
_ '_
/ \
| |
| ____)
| / \/ \
/\/\ (o)(o)| I am Homer of Borg. You will be assimilated
/c \__/--. Resistance ... is ... Hmmmm, Donuts!!!
\__ -------' ___________________________________________
| / \
| | \_____)
| \_____)
| _____|
|______/\/\
/ \
FFMPEG
Try uninstalling and replacing ffmpegthumbs, and possibly ffmpeg also. See if a fresh install makes any difference to the icon display.
Compositor
Is there a setting somewhere to manage compositor latency?
Turn off the display, of disk and folder volumes. Maybe.