Mountain bike vs road bike century ride.?

Is the performance gain over a mountain bike even with slick tires really that big? I have a el cheepo mountain bike, would it be worth to put out a few hundred dollars or less to get a used road bike to use instead? Im in great shape and know I could do it on a mtb, but it will probably take 10 hours. Should i go... show more Is the performance gain over a mountain bike even with slick tires really that big? I have a el cheepo mountain bike, would it be worth to put out a few hundred dollars or less to get a used road bike to use instead? Im in great shape and know I could do it on a mtb, but it will probably take 10 hours. Should i go ahead and dish out a few hundred on a road bike or go ahead and buy the slick tires?
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