It is difficult to answer, as there are many factors that influence loading times. Generally, custom-coded sites, when executed to perfection, should be the fastest. Then again, WordPress has so many plugins for optimization that achieving excellent results there is very easy, especially when the hosting provider is optimized for the platform. Website builders, on average, are deemed to be slower but they also have easily applicable optimizations to boost speed, and well-coded platforms like Ucraft, for instance, deliver quite nimble pages. Keep in mind that this is a generalization, as any site can be very quick with proper care.