i agree that most of the time I see this there are two probable causes:
1. hosts can't evacuate fully
2. storage for where the VIB gets installed is too slow/has issues
The general process is an NFC upload to the hypervisor, then run a command to install the VIB file. It's not all that big either, so internal networks should ship it rather fast.
Most of the time is spent entering and exiting Maintenance Mode (most times when I see it with a customer).
