Manually moving VMs between Hyper-V hosts means that you need to perform a manual (or scripted) export of each VM you wish to migrate, copy the files to the target Hyper-V host, and then import the VM on that host. This process takes time because it requires you to wait until the exporting process is done before you can copy the VHD and other VM files to target host. Once copied, you need to manually import the VM, which doesn't take as much time as the export operation, but still requires time. In addition, the migrated VM will experience a prolonged downtime, due to the fact that it must be shut down on the original host, exported in the shut down state, copied across the network to the target host, then imported and booted.
Note that this article only deals with Hyper-V R2, and not with the RTM version of Hyper-V. Therefore, there may be changes in functionality, and if you're using the RTM version (isn't it time to upgrade already?), you may find that some of the options listed here are not available to you.