The above lesson was something Graham learned in his venture ViaWeb. And guess what, an employer doesn't care if you can create a rich web app in 20 days, if somebody else can create it in 5 days, solving his needs. An employer doesn't care if you're able to fluently speak Clojure, as if it was your native tongue, if some 5th generation system developer is able to create an Ajax web form 5 times as fast as you.
Even today an extremely skilled Assembly programmer can easily outperform a C++ developer, and implement something that's at least 25% faster. However, those 25% are basically the difference between 0.00001 and 0.0000075 seconds. Do you really believe the end user is going to notice it …?
What the end user will notice though, is if you've spent 10 people working for 10 months, or 2 people working for 2 months to solve the problem. This will from the customer's point of view literally translate into thousands of dollars, that the software vendor needs to earn, to make the project break even. In addition, your employer will definitely notice it if your competitor is able to deliver a product into production 8 months before you do.