I've mentioned several times before that when I've opted out, they rope off the WBI machine behind me and begin moving everyone thru the WTMD while I'm getting the hands in the pants. The long security line starts moving very quickly with 6-8 people per minute being processed vs the 1-2 when WBI was the primary scanner.

Based on those experiences, I can say that opting out had a very positive effect on getting people moving through security quickly - it wasn't a problem for other passengers.