Bluetooth audio delay / lag, reason completely unclear to me
Bluetooth lag is a known problem with some, but not all, remote speakers, headsets, etc. We have several Vizio sound bars that have a definite lag, but a Harmon-Kardon speaker set that doesn't. In some cases, adjusting a specific Bluetooth system parameter in Terminal can help with the lag. This works on some of our Vizio sound bars, but we had to hard-wire others to guarantee no video syncing issues.

sudo defaults write com.apple.BluetoothAudioAgent "Apple Bitpool Min (editable)" -int 40

The -int value can be experimented with to troubleshoot, but I'd be cautious and not be too extreme in the values you try.

I found this information at this link and this one.