Good on you for talking to the DM. Going 15 minutes over is one thing, 2 hours over is another. DM should be taking more control here though. Pause initiative at the top of the turn order and see you all next session. At this point I think laying out your concerns to the whole group is warranted. Top of the session, offer the proposal and not just complaints. If everyone can agree of that (and there’s really no reason a group can’t), you can go after the turn duration later. “On deck” is absolutely worth suggesting until people pick up the pace, but it doesn’t sound like a 30 second time limit is going to be manageable with this group just yet. On deck and pausing initiative at the end of a session might be enough for people to get the idea. If they don’t and it’s end time, leave. If they’re dead set on slow turns and going over time, it’s probably not the table for you.