Robert Trent Jones did experiment with building parallel, alternate par-3 holes to speed up play, on one or two courses back in the day, I believe.
That would probably work to speed play, no?
How about multiple tees sideways so 2-4 golfers can tee off on a hole at the same time?
Many years ago, playing Carnoustie, there was a hole with two tees, one on either side of the previous green. Because our mixed group was falling behind, the women teed off on their tee to the right, simultaneously with the men on the left to save a few seconds. (the distance difference from back to front tees wasn't very great back then) I often wondered, with the current speed of play emphasis, if trying this on purpose would help the cause.
It would seem to work best if it was the second of two playing in about the same direction, but could work at other angles with careful placement. And, there would be the question of who takes the cart and who walks to the other tee. It might offer some unusual golf bets, as winner of the previous hole gets to pick the next tee, selecting an angle that suits them better, or the closest walk? i.e., a slicer picks the right tee to fade the ball back to the fw?
It would be at the possible (but highly unlikely) that two balls collide in mid air. That actually happened to me in a High School golf match when I played from across the fairway at the same time as my team mate.