that's a theoretical maximum. you increase Ghz, you have a shorter theoretical maximum length of path electricity can take through your chip. An i7 is not that small. You would have to shrink things further and further to get a smaller chip with shorter paths in it, which is what new fabrication methods have been about.
Obviously this is very difficult. Sure, chips could be even faster if they were just a few atoms across, but who would expect you to do meaningful computation in that size, or to be able to manufacture that.
http://www.google.com/search?q=c+%2F+3.5+Ghz
that's a theoretical maximum. you increase Ghz, you have a shorter theoretical maximum length of path electricity can take through your chip. An i7 is not that small. You would have to shrink things further and further to get a smaller chip with shorter paths in it, which is what new fabrication methods have been about.
Obviously this is very difficult. Sure, chips could be even faster if they were just a few atoms across, but who would expect you to do meaningful computation in that size, or to be able to manufacture that.