Im interested......But dont think i follow what you are saying.
If you dont get the correct flow rate, youd have the parasite pass through the UV multiple times a day but each time it simply gives it a tan, instead of burning it to a pulp, because it didn't make a long enough contact.
If you get the correct flow rate, contact time, it can kill the parasite, so it wont ever need to flow through the UV again.
More times the water flows through can be beneficial to some things but definitely not in a sense for lowering a parasite infestation. Contact time is super important in that regard. It has to be killed by the correct contact time to power otherwise you are just giving it a trip through a plastic housing every so often surely.