Bluetooth transfers audio data in "frames". The sender has to wait for a frame to be done being outputted by an application, encode it and send it over the air. Then, the receiver has to wait for the end of the frame (at Bluetooth data transfer speeds) over the air, decode it and play it back.
According to Ruslan Yanchyshyn on Lemberg the latency for just the Bluetooth protocol part is around 40 milliseconds if the aptX low-latency codec is used; it'll be even more with the standard SBC codec. The main question is :(How is Bluetooth inferior to wired connections?)