I daven in fast and slow minyanim.
Regarding the fast: There are those who blaze along and still enunciate every word, from what I can tell. I assume they have been davening since birth and are not losing any kavana in the process.
Regarding the slow: For me, I come out of such a minyan with such a different and more complete feeling.
I would venture to say the effectiveness of davening is in relationship to the concentration and kavan put into it.
If one is able to “shoot his arrow” of prayer with the same strength in a fast minyan, then fine. If not, then it is not so ok to daven with a fast minyan, unless they need a minyan.