I experience this sometimes (currently running 16.04 LTS), you can easily enlarge the time by:
* high load, have processes running taking 100% CPU
* a lot of used RAM
* slow I/O (hard disk)
E.g., on a netbook, I was regularly even able to type in commands on open shells, start or close programs.
It still happens on a decent machine, but less often and with much shorter "flashing".
I experience this sometimes (currently running 16.04 LTS), you can easily enlarge the time by:
* high load, have processes running taking 100% CPU
* a lot of used RAM
* slow I/O (hard disk)
E.g., on a netbook, I was regularly even able to type in commands on open shells, start or close programs.
It still happens on a decent machine, but less often and with much shorter "flashing".