Hey now!

I'm butting in here, right now, to support the notion of harmony singing.

And who sez harmony singing ipso-facto slows the tempo? Not when I sing harmonies, that's for sure; I can sing just as fast or slow as circumstances require.

In a choir, you're not really free to extemporaneously harmonize; you're pretty much required simply to sing one written part or another. Now, singing as part of a church's congregation, that's different ~ you can sing just about however you wish (poorly, adequately, or brilliantly), and no one can really object.