It actually depends if you chose "rolling pool update" or "Install all pool patches", as you're talking about evacuate I assume you went with the first one.
Rolling pool update (RPU) is documented here and Pool updates here.
But to sum it up, "Install all pool patches" button will indeed run the yum update on all server, so similar as doing it manually, while RPU will do hosts one by one, moving VM to other hosts in the pool, install updates and reboot host. Therefore it can take way longer to complete, time will vary based on the number of VMs that have to be migrated around, network speed between hosts, etc…
RPU is the recommanded way as it allows hosts to restart and therefore take hypervisor, microcode and dom0 kernel updates into account right away with no service interruption. But if you don't really mind shutting down some of the VMs to restart hosts, or if there are no low level updates that requires a reboot, you could get away with just the yum update manually. But if the RPU is started already, I would not advise trying to do things manually at the same time.