jr and rpc...I guess part of my idea in advising a 15mph minimum speed for bikes over this bridge, and watching for big gaps breaks in the traffic, is to not slow down traffic on the bridge any more than necessary. Maybe some people are comfortable with it, but a bike poking along at 10mph or less with 4-5 cars backed up behind it isn't a situation I'm comfortable being in.
For bikes to be doing this regularly doesn't seem likely to make any friends with the motor vehicle crowd either, especially if those motor vehicle operators sense that the person on the bike isn't making an effort to move things along. Each to their own though. Not to say that you two are advising this, but I'm aware that some people traveling on bikes believe motor vehicle operators should just 'suck it up' upon being presented with vehicles on the road traveling very slow.
15mph is 50 percent faster than 10mph. 20mph is 100 percent faster than 10mph. True, though those speeds are much slower than 30mph and 45mph motor vehicle speeds, they can make a big difference in enabling higher volumes of traffic across a span like the St. Johns Bridge. I think this is a simple reality appreciated by many people on the road.