Does the engine seem to go flat, or does the engine rev fast but the car accelerates slow?
If the second, could be a slipping clutch. My usual test is 3rd or 4th gear, about 3000RPM, slight uphill is even better, and then stomp the gas. The revs gets you into the high torque range, the gear and hill load the engine more. The result is asking a lot of the clutch and it will slip if it's failing. Thus the tach climbs quickly, the slowly moves faster.
Any engine lights on the dash ?
Could be a slightly blocked cat converter if it’s not the clutch slipping.
I’d do the rounds anyway and clean the maf sensor, clean the plugs, clean the throttle body, maybe even reset the ecu.
Is this a sudden thing, always been there, or slowly getting worse?
Does it roll easily on flat ground in neutral?
It could be a dragging brake (that would hurt gas mileage as well). My test is, a few miles on a highway at speed, find a longish uphill, coast up it in neutral and,pull to the side. Get out and check the center of each wheel by hand.
Any wheel noticeably warmer than the others is likely a dragging brake. Figure out why. Could be a stuck piston, stuck slider, etc.