I think its what they call compression loss. The revs have to be higher to give sufficient cooling. On a healthy compressor, its should easily cool down the cabin even at idle. You need to hook it up sa pressure gauge to confirm.

When you replace the compressor, kahit surplus is ok. as long as its quiet, and has good readings sa gauge. Replace the drier and expansion valve as well, have the evaporator cleaned then flush the system to get rid of debris.