As a waitress, bartender, server, caterer for years, both in public and private venues. You want the people you are serving to continue what they are doing when you serve them. I always believed that was the sign of a good server, "to be invisible".
When I place a plate in front of someone or refill their water, If they say "thank you" or they say nothing, I am doing my job. The waitress should have put the plates down and walked away. If she needed to ask the patron something like, "do you need mustard?" she could come back.
Frankly if I had a customer start to pray as soon as I put their plates down, but before I put plates in front of the other people at the table, I would have thought they were being considerate of the non prayers at the table. This way they are finished with their prayer by the time everyone has their food and is ready to eat.
As for loudness I think that is how relevant to how embarrassed you are.
But of course that is way to much over-thinking...I really would have forgotten the entire episode by the time I went to the next table.
The best time to plant a tree was 20 years ago, the second best time is today.