I'd been riding a Trek 520 for 20 years, based on the original simple bike dealer "fitting" and very few tweaks. I set up my cleats for SPD pedals by following online guidelines, put on a new seat and added a bit of tilt, etc. Did some commuting, some multi-day touring, lots of 50 mile+ rides and pretty much felt fine.
This year I bought a new high end (Trek Domane) bike that came with a fancy Retul fitting session. I first did the same old bike dealer "fitting" and rode the new bike for a month. Different geometry from the 520, plus different gearing: 2 x 11 vs. 3 x 9 on the 520. The Domane is a good deal lighter than the 520, going up hills was noticeably easier but on long, hilly rides I was feeling it in my knees and legs where I never had that on the 520.
So, did the Retul fitting session and the bike fitter moved my cleats all the way back on my shoes and raised my seat by about 3/4 inch. Went back and did the same hilly long rides and huge improvement - back to legs just just feeling tired, not sore, just like on the 520!
I can't say that the position change made me any faster, too many variables. But, it definitely made a difference in efficiency, that either translates to higher speed for the same effort or less effort for the same speed!
John P.