Car is having acceleration problems

2000 plymouth neon my car seems to not want to get up and go it slowly is getting worse, cant seem to get up to higher speeds seems bogged down and is taking longer and longer to get up to higher speeds, 5 speed manual transmission doesnt seem to be slipping no back fire or rough engine just seems to be working really hard to get up to top speeds and sometimes after driving for a while can smell like burnt gas was wondering since it is 11 years old could this be the catalytic convertor becoming clogged have a 1998 honda that did same thing and got to where I couldnt get it over 20 mph and that was the cause could this be same thing or is it something else.