https://software.intel.com/en-us/blogs/2014/02/19/why-has-cpu-frequency-ceased-to-grow

All of you probably recall the rapid rate of CPU frequency advancement at the end of the last century and beginning of this one. Tens of megahertz rapidly transformed into hundreds, and then hundreds of megahertz quickly became a full gigahertz, then a gigahertz and a bit, finally two gigs and a bit.

However, in the last few years, we’ve seen the core CPU frequency growth has slowed. The 10 GHz result is still as unreachable now as it was five years ago. Why the slow down? What is the obstacle for an increasingly expanding rate of frequency?