I am using an (older) nvidia grahics card with the 390.87 driver (single graphics card) on a desktop machine with the latest-testing fedora 29 updates and plasma5 desktop.

I noticed, that when i switch over to a virtual console (textmode) and going back to graphics mode (Ctrl-Alt-F1) the screen is properly redrawn, but any input like mouse clicks for context-menu or dragging a window do not work any more.
This problem/freeze stays for about 15 to 20 seconds, then all is back to normal.

I created a second useraccount just for testing purposes (for fresh configs) and tried the same: After returning from textmode, the gui has a problem and indicates this in a popup in the taskbar:
"desktop effects were restarted due to a graphics reset"
and all is fine again - this takes way less than the above 15-20secs!

Another test with the former, problematic account never shows this popup at all. I tried again, this time disabling the compositor (Alt-Shift F12, no more shadow-decorations on windows for example) first.
Returning from textmode gives an immediate usable gui again, although without compositing effects. If you now switch them on using the Alt-Shift-F12 shortcut, then the 15-20second penalty hits you again...

I discovered, that in those 15-20seconds the process:
"/usr/libexec/Xorg -nolisten tcp -auth /var/run/sddm/{b67ef872-33b8-42a5-8a0b-0b8f681c717d} -background none -noreset -displayfd 17 -seat seat0 vt1"
took on between 60 to 75% of cpu time (on a 8 core machine) for the exact 15-20seconds waiting penalty - something that did not happen with the previously mentioned test useraccount (and switching from text to gui)

Questions:
- what kind of logfiles can i provide to narrow down the problem? 
- is there any config files we could tweak, so that the gui recovers more quickly? (see testuser-account above and the popup-hint)

Good thing this is easily reproducible, and besides switching between text/gui i haven't seen any other defects - so i don't think of a configuration problem. Moreover, this switching has worked before without any time-penalties of the compositor....

Any help is appreciated