Tires for 28' city bike (622x19C) to reduce rolling resistance?
I have replaced 2.0 (50mm) Schwalbe Marathons for 1.35 ones, and that made an ABSURD difference in my roling resistence. I was feeling burnt out by commuting, and since then I started to feel "exercised", getting to destination much faster and/or with less effort.

Then I got a bike with 700x23 tires, and THAT is good, concerning speed and low effort.

The only thing I think is very important: these skinny tires work fine, even with not-ideal road condition, if the bike has a more performance-oriented position (a firmer grasp on the handlebar - which is lower - , less weight on the saddle). If you take a more upright citybike (the first bike I described), the extra speed gained from thinner (not skinny) tires can make the bike too uncomfortable if road condition is not good.

I have also heard those theories about "balloon is the new fast", but that seems very non-scientific, since the "studies" never describe the parameters that varied versus those that remained constant, not to say other methodologic steps. As for me, narrower is obviously always faster (which doesn't mean always better).

Hope this helps!

(by the way, I think 32mm is a good number to try)