A/C only works in the city.

After digging around for a while with no luck, here's my question. The air conditioning in my 2005 Prius (125,000 miles) runs great at city speeds. On a very hot day it may take a bit to get going, but generally it is ice cold. The problem is on the highway. After driving at high speeds for 45 minutes or so, there's barely any air flowing out at all -- even if the fan is on the highest setting. What does come is not very cool at all.

This problem first occurred last year. During the summer, we make many trips to hang with family at the Jersey shore - about a 4 hour drive from DC. After one or two experiences sitting a sweat box for 4 hours in 90 degree weather, we asked our mechanic and he told us there was a clogged filter. After he "unclogged" it, things seemed to be better. And once September rolled around, we were back to mainly driving with blissfully cool A/C in the city.

We just made our first trip of the 2018 season, and the A/C puttered out on the highway once again (it was about 80 degrees). Could this so-called filter already be clogged again? Or is there something more significant going on here?