Take Pentium 4 - which used the architecture Intel was working on when they were predicting that they'd have 10GHz CPUs by now. Passmark's score for a 3GHz Pentium 4 is 384.
An i7-3940XM runs at 3GHz, and gets a score of 10,490. That's a quad-core computer, so I'm going to go ahead and be sloppy by dividing by 4 to get a score of 2622.5 per core.
In other words, a single i7 core is doing almost seven times as much work per clock cycle. And assuming they only ramped up the clock rate, the P4 would have to be running at over 20GHz to match what the i7 is doing on a per-core basis.
Given the heat dissipation problems that come into play with high clock rates, that seems like a doubtful proposition. In hindsight, it looks like Intel definitely made the right choice by dumping the NetBurst plan and instead letting clock rates stagnate (they've even retracted a bit from the high water mark) while designing the processor core to do a lot more with each cycle.