2002 Kia Rio Car has lost it's get up and go!

Engine Performance problem
2002 Kia Rio 4 cyl Front Wheel Drive Automatic 142000 kms

My car has recently become very sluggish at high speeds, when you get on the highway if you leave your foot down to accelerate quickly you only get up to about 90km and the tach sits at almost 5000 - it won't pick up anymore than that but if you back off the pedal the tach will drop down to 3000 (normal for highway in this car) and you will accelerate all be it very slowly. I had it in at the dealership last night for an oil change and asked them to look at it - their guess is that the exhaust is plugged up and the pressure is not releasing but they were getting ready to close so no time to investigate further. So today I took it into a friend who owns an exhaust shop he said based on the temperature readings along the exhaust that the exhaust is working exactly as it should and it must be a fuel problem. About 3 months ago I had the fuel filter and spark plugs replaced as regular maintenance.