If the compressor is getting weak, it will need to spin really fast, like at highway speeds in order to compress the refrigerant enough to cool the vehicle. Get a second opinion, maybe all you have is a clogged condenser, or you may just be low on r-134.
I had the same probglem with our 2005 Freestyle. My wife found a site on the net about freestyles www.myfordfreestyle.com had a question about air conditioning. There is a part in the compressor that cost me $40.00 and I had a shop put it in and the air is fine. Ford told me I had to replace the whole system for $1800. This is the site I went to to get the part. It is called a control valve. Cost about $180 for have the shop drain and recharge the system. Hope this helps.