Intake/Exhaust valves
And they still suck after a port job, which is the reason that the forced induction engines are a good way around this problem. Trying to make horsepower with a normally-aspirated slant six is like trying to run a marathon with a piece of duct tape over your mouth... breathing is just too restricted.
Of course, everybody doesn't want, or need to go tens or even twelves, and if you can be happy with a fourteen second car, the smart thing to do is build a normally aspirated engine that has a good cam, high compression, plenty of carburetion and a nice set of headers, and avoid the complexity and tuning issues that always come with a turbo or supercharger. Probably not as cheap, but waaaay easier, and that's surely worth something!
A fourteen-second car can be a lot of fun!!!