4GHz seems to be the soft limit with x86 processors. Ten years ago, the single-core Pentium 4 stalled just shy of 4GHz. Today's high-end Core i7's also have a

4GHz seems to be the soft limit with x86 processors. Ten years ago, the single-core Pentium 4 stalled just shy of 4GHz. Today's high-end Core i7's also have a stock frequency around 4GHz, though of course they're at least ten times more powerful. AMD tried 5GHz at one point, but that was just a gimmick and most of their current models stick to 4GHz or less. Despite having radically different designs and process sizes, roughly the same frequency remains the point after which energy consumption just runs out of control. Performance-wise it doesn't matter because we've gone multi-core and found ways to get more work done per clock cycle, but I wonder why the top frequency seems to be so consistent.