Blackberry,
I've noticed the same things. Those that sound like they are struggling could need oil, or it could be the motor, ~or~ the belt could be too tight.
When I was cleaning and refurbing the Featherweight I got for my wife I put a new cogged belt on it. I had it adjusted where I thought it should be, tightness wise, but when I tried to use it nothing happened but the motor grunting, groaning, and buzzing. It had run fine with the old belt. So I loosened up the belt. I had to loosen it up a lot.
Once it was looser and the motor could turn it, it felt as you said laborious until I got the belt tension just right. After that it runs just fine.
Joe