Here is a story that happened to me with my 2007 civic with 48,000 km on it.
The car is driven 95% of the time by my wife. It is her car. There is a company owned Attock Petrol pump near my place so 99% of the time fuel was filled from that pump. On day I was driving the car and I was standing next to a Civic similar to mine. When the light changed, I tried to get ahead of the other civic but no mater ho hard I pushed my car, the other one just flew away at an incredible rate of acceleration. At least that is how it looked to me in my "slow" car. A few months earlier I went to Lahore via the motor way and while the wife was a sleep in the passenger seat, I pushed the pedal all the way down and was a bit surprised to see that after 130km/h there was no more speed to be seen.
Being a bit concerned, I took the car to Honda Classic. The service advisor took the car for a test drive, did a few acceleration runs with high RPM gear shifts and said the catalytic converter is blocked. Long story short; the cat con was "cleaned" and the car is back to its former power. Nothing else was done. No "washing" throttle body nonsense, or plugs or fuel pump or anything (fuel filter and pump changed at 40,000km as stipulated in the service booklet). I must add that my car is totally 3S maintained and all service done at the right mileage and the booklet is fully stamped by the dealer from day one.
You are welcome to try out the other suggestions. I am guessing it is the converter. BTW, since this episode, we have changed to Shell and I do an Italian tune up once or twice a week whenever I am driving alone . That should keep the cat con clean.