If it was a trans issue(assuming automatic) you would probably notice the RPMs increasing but not the speed, however a trans issue is not completely off the table.
Assuming you have good top speed, i.e. no overall power loss above 50mph, then I would get a volt/ohm meter and check the TPS(throttle position sensor).
It has 3 wires connected, center wire is the "return voltage" wire.
When key is on(engine can be off) the computer sends the TPS 5volts(top wire).
The TPS is like a volume control or light dimmer, a variable resistor.
On the Return wire if throttle is closed you should see .7-.9volts, under 1volt
With throttle wide open you should see 4.6-4.9volts, above 4.5volts
In between you need to see a steady increase or decrease in voltage, no jumping around, a TPS can get a "dead spot" this would be seen as loss of power at that position and changing the gas pedal position(TPS position) would cause a return of the power.
Set volt meter to DC volts, 20vDC if available.
Get a sewing needle/pin, use it to pierce the center wire
Put meters black probe on Ground(battery is fine)
Turn on ignition key
Put red probe on needle/pin
You should see under 1v
Manually open and close throttle while watching voltage on meter.
FYI, a volt/ohm meter costs about $25, cheaper than pretty much any sensor, so........that $25 can save you hundreds of dollars in the long run, and it works on any vehicle, so truly a universal tester.
General power loss at higher speeds is usually low fuel pressure or clogged exhaust, this doesn't effect a "limited range" i.e. it doesn't get better at higher speed, it gets worse.
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