2002 Toyota Corolla Performance

At the 1st maintenance of the car the mechanic put in the wrong plugs in the car and I got engine problems and the o2 sensor light came on and the a/c use seem to strain the engine. I changed the sensor/plugs and now the car seems to be moving sluggish. Multiple times the mechanic has done diagnostic and the only problem was the o2sensor. I had the car for four years and has only changed oil/filter/T. Fliud/air filter/plugs and removed the catylist converter. The timing belt needs to be changed and I wonder if the car has a gas filter that needs to be changed also? What can I do to get back the best performance from the vechicle? It just feels as if the engine is being held back. HELP.