What's the minimum cruise speed of modern airliners?
It's called the stall speed. Below which the wings won't give enough lift to stay airborne.

Another minimal speed to adhere to is the minimal control speed. Below that the controls surfaces won't be able to counteract suddenly losing an engine.

However slowing down before landing has nothing to do with waiting on their turn/clearance. If they need to wait they go around in a circle instead.

The slowing down is so they touch down with as low speed as possible to limit the braking distance needed on the runway.