I can't understand the exact cause of it, however this bug causes beefalo to slow down to about the speed of a player.
From personal experience it can occur anytime after initial domestication if you enter and later exit the caves. I didn't record the incident like usual but found a video elsewhere on the forums that is identical to the one I had.
YOU CAN SEE MOVEMENT PREDICTION IS ENABLED IN THIS VIDEO AS WELL.
This isn't the same tendency as this is an ornery beefalo while the issue occured to my rider, so I assume it occurs to all beefalo. It, however, never occured any other beefalo. Just the one I had began taming.
I did come out of the caves before it happened, it was as fast as normal right before I went down there.
Twice this has happened and each time it is the exact same thing.
Start taming beefalo, Ride beefalo to sinkhole, enter sinkhole, get light bulbs, exit sinkhole, ride beefalo.
At this point the beefalo is about player speed.
Even after doing this on a local server the beefalo seems to acquire a slower speed though it is not as severe.
The bug is caused by lag compensation being set to predictive as is evident here.
Toggle lag compensation to predictive and beefalo is slow.
Toggle lag compensation to none and beefalo is normal speed.
To fix beefalo speed remove saddle with saddle horn and place it back on.
If beefalo has not yet been domesticated you can also allow beefalo to shake off saddle, then place the saddle back on.
Have Lag compensation on, Start domestication, Enter and exit caves, Ride beefalo. Is a hit and miss most the time.