I’ve seen this on a FP3 for a few versions, but apparently from v1.13 → v1.14 the behaviour is now gone if you enable the screen, it took ~30 minutes. It did take significantly longer before (as in multiple of hours plugged+screen-enabled too) as described in the link.
There was a non-public gitlab issue as to the screen on/off behaviour as commented at https://gitlab.e.foundation/e/backlog/-/issues/5701#note_295656 - there’s no filed issue on general duration.
The ota updater is doing ahead of time optimization of dalvik executables. If it’s in the multiple hours for you a logcat might be interesting - run “adb logcat | tee logcat.log
” during apply-update.