My most recent transfer to the EU from SIN on J on the 789, they held the line for J, and boarded all the Y pax first. When the J bus arrived, then they boarded the J pax while the Y pax were all waiting in the Y bus, which must have been about 20 minutes, I'd imagine.

Note: My flight from SIN was delayed quite a bit, and I thought there was a good chance I may miss my connection. When I went through the DOH J security, I think the security personnel was about to search my bag, but I mentioned I had very tight connection, and he just said, "ok, go," and waived me through. Can anyone imagine this happening in the US or any EU airport? In LHR, if one forgets to place toothpaste in his bag, it's pulled for secondary screening. I literally ran with my bag all the way to the gate and just found the J line with Y already boarded and was wondering what was happening for a moment.

PS - When you arrive at the plane, they disembark the J bus before the Y bus.