There can be. The introduction of CAT 6 cable was done to meet the specific needs of "Gigabit Ethernet". A whole generation of new networking components has been introduced for the purpose of having a Gigabit network. Network Interface Cards, Routers, Switches, Patch panels, Hubs, and many other components have been developed in order to create a network that can be "certified" as a full gigabit network.
It is important to remember that in order to achieve this goal, EVERY single component in the network must be Gigabit certified. If a single component in the network is rated at a speed below that, it is possible and very likely that the network will operate at a much slower speed.
Many IT professionals and installers have realized that very good network performance can be achieved using Cat5e ethernet cable and other networking components. A good qualityCat5e cable can deliver near or at gigabit speeds, and will do so at a much lower cost thatCat 6 cable.
The most important thing to remember is this: Buy what you need, and no more. It is not necessary to spend the money for CAT 6 if you do not have to.