Posted but no reply want to see if anyone can help

First found AC won't turn on until accelerated to high speed, then even high speed wouldn't work, until about 15-20 minutes,, sometimes after reduce speed, and suddenly with a hssh sound it's on. After turning on, seemed the higher the car runs, the cooler the air is. Went to mechanic, said Freon was only 1 lb, suggesting me change to R134a, agreed, but after that same problem. I felt the belt a bit loose, but the mechanic said it's ok. but problrm still there. I told mechanic there are engine speed switch and shut off switch in the ac, he seemed didn't know. He added less R134a, is that a poosible reason? But problem exact as before, also after ac on, if reduce the speed, it can turn off again and still hard to start up. And also seems cold weather easy to turn on ac, but the hotter the weather, the ac more didfficult to turn on. Any help would be appreciated.