The only fight that takes 4-5 min in a 1v1 that I know of are either earth and/or water main fights where both side use a grindy team and it actually take the full 10 rounds or close to it. Most fight doesn't last nearly that long even when evenly matched in terms of power. To be honest, I'm not even sure if a full 10 round fight last quite that long, I'd guess it's more like 3 minutes. But as I've never used a grindy team myself, such long 1v1 fight is rare for me so I can't be sure.
I think it might in fact be interesting in terms of tactical choice player makes. Do they use a slower team that will more likely get them a win (water immortal band wagons) or perhaps a faster team that is not as reliable but gets more fights?
Granted, blitz right now is still quite effective. Ideally, this would be best if blitz aren't as reliable so that a choice between reliability and more fights must be made.
On a side note, power gap in group one would still be huge. It just might go from 50k+ (where + goes from 80k to 190k) to maybe 60k+. There aren't THAT many 50k+ players who aren't lv 90, so they won't bump out all that many "low powers". So there's still going to be a huge gap there, fights for most of the ptw would still be pretty fast one way or another (on a side note, once about a time, the bottom power of the bracket would have been higher, but some of the bottom of group one are quitting, either just sage or the game altogether, thus lower powered players are getting in there)
For the rest of the brackets thou, yes, fights would slow down. I'd say the mid 40ks to mid 50ks might be the slowest? I'm not sure. Wherever the most player of similar power are I guess.