Buffering is frequently associated with insufficient bandwidth (i.e. a poor download speed) but turning on your Bluetooth speakers shouldn't affect that. (You still use the same amount of data to get the video to your computer, irrespective of whether you're then forwarding the audio to a Bluetooth device or not).
However buffering can also be associated with insufficient resources on your computer. Using Bluetooth can create extra demands upon your computer's processor, meaning that it's left with insufficient resources to be able to display the video properly. That's most likely to happen when something else is already hogging your computer's CPU (and/or its memory), such as anti-virus software that's got stuck in a loop.
A look at your Task Manager (which can be accessed by pressing Ctrl, Shift and Esc all at the same time) can sometimes provide some clues as to what might be using lots of your computer's resources but it's not always easy to find out what's wrong or, if you can, to work out how to fix it. You might just have to live with the problem and either use the sound direct from your computer or use wired speakers instead of Bluetooth ones.