Unfortunately audio lag is a known major issue with Bluetooth and usually comes down to the fact that the source transmitter has to re-encode the audio into the A2DP (the Bluetooth Advanced Audio Distribution Profile) format before transmitting. This can be quite hard work for the transmitter (the PC or phone), and coupled with the delay in receiving and then decoding the audio before playback on the Pi, this is what will be introducing the lag.
Because of this, as Paulo mentioned, a lot of players now support audio sync delay to ensure that you can match up the audio lag with the output video. Unfortunately, with the likes of stream players over the web, these are designed to stream to a single renderer (the PC/phone/etc.), rather than the renderer then passing audio or video onto a different renderer, and because of this do not include sync options.