Will the Kevlar boat be faster than the fibreglass boat?
Not enough to be very noticeable. Unless you are an avid racer, the best reason to buy a Kevlar kayak is the advantage of lighter weight when loading and unloading onto your car and carrying the kayak to the beach. If these reasons are not an issue, save yourself a bundle and get a fibreglass kayak. You can get some really nice accessories with your savings!