don't know much about kayaks - we do an annual multi-clubs trip every March that includes all kinds of boats - short kayaks are always slow (some are really short ww boats, others rec boats).  most everybody with a kayak has some sort of touring kayak/sea kayak in the 14' range - they all do well enough.  Most are plastic and they are quite heavy (we do a bucket brigade "portage" over a couple of dams - they get really heavy towards the end of that.

You might want to look at Paddling.net reviews to get some ideas;  likely, you are better off buying a composite boat used than buying a really heavy roto-molded one.  You can search the classified on P-net while you'r there.