You might find reviews that quantify it but yes, slower writes.
Once a month backups, I'd sooner have an external that powers off rather than always getting power. I have one plugged into a hub and power strip, each having a switch to toggle rather than having to plug/unplug an external, or reboot into bios settings to put an internal online... but i use it more often than monthly, if it were only once a month, plugging and unplugging would weigh against the cost of the switchable hub and power strip, except I already had those.