I have a practice amp with a bluetooth input and it would be great to use the songs app on my tablet through this, but due to the latency in the bluetooth connection there’s a short lag between the visuals showing the chord to play and the audio (probably about one beat at ~130bpm so enough to put you off).
It would be nice to be able to adjust the video/audio phasing to compensate for this.
Unfortunately, the “lag” (officially called latency) of a Bluetooth connection is variable and can’t be adjusted for. This is why wireless guitar transmitters don’t use Bluetooth; you’d pluck the string and the note would have a significant, but unknown, delay. The amount of delay would be different every time you re-connect via Bluetooth and/or as the the environment changes (turn on microwave, etc.)
In order to keep the video and audio in perfect sync, you need a cable or a much more expensive wireless transceiver.
The Bluetooth connection is really good for playing an audio-only backing track through your practice amp, though 
Thanks - I hadn’t realised the latency wasn’t constant for a given setup. Back to cable it is then…