What are the consequences of turning iCloud Document and Data syncing off?
The consequence is data stays the same on your Mac and isn't kept in sync with other devices that use iCloud sync.

You can test if this is the cause of slowness by just disconnecting from the network.

Turning off iCloud documents is simple though and you can always turn it on again if you find something else was causing the slowness or you wish to have an easier time getting files back and forth.