Record the times of the groups on the course and keep an eye on the pace of play.
All groups of a round at a glance: how many minutes each group is ahead of or behind the target time and at which hole it was last measured – green is on time, red is too slow.
On the phone the marshal picks the hole and the difference to the target time, optionally with a note such as "had to wait". Every group shows its target time per hole and all entries so far.