I dunno. I used to think a few watts makes zero difference...but I've sort of changed my tune - a bit. I started looking around for new tires at the end of last year. It became a bit of an obsession...I researched every single pavement tire on bikerollingresistance.com...and ended up switching from basically the slowest touring tire (Vittoria Randonneur) to the 2nd or 3rd fastest touring tire (Vittoria Voyager Hyper). It was a difference of 10 watts or so.

It's definitely noticeable. Not mind blowingly faster of course...I had on a no tread tire previously, it wasn't like I went from a 3 inch off road knobby to a racing tire. But I definitely felt it. Or...placebo effect from spending so much time picking out a tire? Who knows lol.