is it okay to daven with a fast minyan

I don’t understand your question–If you think davening is lip service then go ahead…but as far as I know, tefillah is a very serious thing, it’s b’makom karbanos. So imagine if you were bringing a karbon to BH”M and would fly like a speed train then yea…but I dont think you would.

The problem arises is when people get used to doing something. Even the Kohanim had to switch off while doing avoada not to get used to doing it.