Code p0420, loss of power and high fuel consumption?

My vehicle listed above consumes a lot of fuel and loses the power to drive. RPM goes up when I accelerate but the movement is slow for a few seconds then back to normal. RPM goes up during hilly roads. There was a check engine light for a few days and it went away by itself. I have an OBD scanner that detects a p0420 error if I accelerate more and drive. If I drive normally then there is no error. Also, when we have the P0420 error code, I noticed the O2S11 voltage is 2.255. Can you please explain to me the cause and how to get back the fuel efficiency and power? Acceleration is very rough and slow when we stop and accelerate again.