I'm not exactly an expert on that subject but there is no way to tell if there's a limit. If you see that question on it's literal meaning, probably there isn't a limit to CPU speed when it cames to performance meanings, like manofchalk said.
There's a very good and conductive material called graphene, which consists solely in carbon (its a carbon isomer). Its a very fragile material, so the scientists haven't find a way to use it to create CPU's yet. But that's just a matter of time. It is estimated that a CPU build with graphene would be five hundred times faster than any known processor. Still, it is most likely to be used as warfare technology and researches before becoming "domestic", and THAT could take years.
Also, not all the CPU's are electrical. For example, our actual processor's are electrical since ever. Pentium I, II, III, IV, Dual Cores, Core i3 to i7. They all work on electricity. That not necessarily prevents them to be faster, but since they work on a binary logic (0 and 1 as with or without eletric current) they MIGHT have limited development over time. And that's one of the reasons why, all over the world, scientists are already creating atomic computers. These processors no longer work on a binary logic, since atoms might have infinite positions over a very small amount of time, they are using it's "spin" to create CPU technologies that are hundreds of thousands faster than graphene.
These computers already exists, but they are on the size of an entire room. Also we don't know the atoms so well, and it is mostly instable.
But, since the first computers in human history used to occupy entire buildings, i believe that there will be a day when a atomic processor fits a machine of the size of a smartphone, maybe even smaller. I'm just not sure if we will be alive until than. But these processors, just as the graphene technology, shows promise.
Some speculates that it would be just as fast as a human brain, or even more. That's is hard, but possible. Which creates a dilemma of how dangerous could this be all over the scientific community. Just as manofchalk said, there are chances of machine rebellion. This can be seen as a joke, but also, it might not. I don't really remember which, but there's a University (maybe Stanford? Google It and you'll see) that created a research department to prevent and discuss machines over-intelligence that could put our existence to risk. That's crazy, we all know.
I hope that this give you some clarification.