Even several pounds of difference is still likely to be only a few percent of the total weight you're moving (bike plus rider.) Unless someone GIVES you lightweight wheels, I honestly wouldn't bother. You can get a decent, serviceable road bike for well under $100 on eBay or Craigslist these days, if you don't mind riding something that's 20 years old. A 20 year old road bike will be faster than the OP's mountain bike, even if you dropped pounds and pounds of weight from it.