I'd start with the fan clutch. With the engine warm, you should be able to hold the fan still or slow it down greatly (use a cloth to rub lightly against the blade tips) with the engine idling. If it resists strongly and will not slow, the clutch is bad. An engine fan running at 4000 RPM can easily pull 15 horsepower off the engine which will slow your acceleration, but not necessarily diminish your cruising speed.

Beyond the fan clutch, I would look to a restriction in the air intake, then the fuel filter or fuel pump as the next possible issues. If any of these were bad though, it's unlikely you could reach maximum shift point RPM levels. Will the engine go over 4000 RPM on a full throttle upshift?

My experience with a bad catalytic converter on other vehicles was such a severe restriction in the exhaust system that the vehicle would not exceed 30 MPH, so I doubt that is your situation.

I suspect that your extended idle period encountered a failure point in your fan clutch. Good luck.