That depends on which motor it is and if it's adjusted properly. Mine was a late 90's Evinrude 25.
If it goes past horizontal, it can make a VERY noticeable difference because the edge of the throttle plate "scoops" air and disrupts the venturi that pulls the fuel. Depending on the carb design, of course.
Not trying to be snarky or combative, but it is definitely a thing.
Someone on this forum told me the same thing many years ago, and it solved my problem. I can't remember, but I think there was a max throttle screw that was missing or adjusted wrong. That, or the roller was adjusted by the PO so the plate never closed fully because it needed a carb cleaning. Either way, it was an easy fix. It didn't gain me any more speed, but I didn't have to feather the throttle to get WOT, just crank the handle and go.