Aircon poor and lacking power/missing when on ?

[Gilby]

My aircon has never been great. Recently had it recharged, new pollen filter (it was smelly) and still wasn't great. Now the engine has started to misfire/struggle when it's on.
Am I looking at silly money to have this investigated or are there some obvious things to look for? My initial thoughts head towards the compressor pulley not turning freely.
As always, advice greatly appreciated.