Look for an exhaust restriction- - - - -Loosen the pipe flange where the exhaust pipe meets the manifold- - - -about a 1/2" gap will be plenty. I've seen a few mufflers packed with acorns by squirrels or chipmunks, and the vehicle won't get out of its own way. Take a test drive and see if the speed improves. Ditto for a restricted air cleaner. Engines have to breathe to make highway speed.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!