Could need cleaning and/or adjustment. You can try a dry spray lubricant, however if they are that difficult to operate that would be an indication of some other problem IMO.

+1

Never met a vinyl window that was taken from tough to operate to easy with lubricant.

Typically a hallmark of a poorly installed window.

clean the tracks then try silicone lubricant spray on the tracks, don't use petroleum based lubricants since they attract dirt. if they are still hard to operate then it's a possibility it was installed out of square etc.