1999 Limited convertible air conditioner works but not very cold

I have a 1999 Limited convertible and the air conditioner works, but only well when driving at highway speeds.  The whole system was replaced two years ago, and the system never has performed that well.   Is there a sensor or temperature control that would keep the a/c from working well at street speeds?  The car at idle never increases RPM when the a/c is running.

this is all guess work untill u can tell me high and low side pressures when running and how much freeon is in it. A sensor will stop it from working all together

Also who replaced the system? Was it a friend or a licensed mechanic who deals with AC? Was it an aftermaket system he might not have enough freon in it.