I also don't get this idea that you will struggle with making cities quick as Progress, it has never been my experience. If anything, the +3 food from the Fraternity policy allows you to use your secondary cities instead of your capital so it's potentially even faster as Progress as long as you improve your luxuries fast enough, which is easier to do anyway.
I have done 3 experiments where I tried the exact same start, one as tradition, and one as progress. The results are noted in the Strategy area, but the general summary was that Tradition is faster at cities 2-4, they both caught up at around city 5, and then progress is faster on cities 6+. As noted above, with progress your secondary cities become "settler producing" much quicker than Tradition cities, so a good progress player shifts to using secondary cities earlier to offset the weaker capital for settler production.