They can be as fast as the speed of the components you can fit into them. Each year they can be faster, so eventually you can get a tablet size system that is as fast as a current full size low end gaming laptop.
If you are talking about right now, if we don't care about weight or thickness, you can get a system that will run the most demanding games at least on low settings. Most issues with size is that you need a way of dissipating heat, and right now there are not a lot of ways you can replace a heatsink which needs space for a fast video card and CPU.