You seem to imagine every conflict starting with planes within shooting range.If you are talking F-86 vs. Spit IX, then sure, every time. P-51 vs Spit IX, the speed difference isn't enough to offset the other advantages which for the Spit also includes climb as well as all the things that comprise maneuverability. And crucially, in dive, they were basically equivalent. I'm not sure which one had the higher dive speed but per my unanswered question upthread, once you are diving at 500 mph it doesn't take long to hit Sea Level.
Maybe the P-51 can force or refuse combat more often, but what happens when they do engage? They can try a very safe strategy of hit and run only in optimal conditions but they can't protect bombers (or defend enemy bombers) that way. If they take losses with every serious engagement the operational advantage of being faster is basically checked.
A P-51 that decides to disengage can try to pull away with that 30 mph speed advantage, but that means they will probably be in range of the guns on the Spit for enough time for a burst or two, and that sure is a risk.
Speed is an advantage, but it's not the only advantage. Only an idiot thinks it's that simple.