I would find it hard to believe that c++ can be that much faster for many websites since the bottleneck would be the I/O subsystem. I don't understand how they are getting this 40x speed up. I wonder what the benchmark looks like.
This is easy enough to test with ab (apache bench), siege, or jmeter, by just cranking up the number of concurrent requests. I've seen 100x differences in implementations of web servers before, 40x for python vs c++ is pretty believable.
Today you can get 15MB/s to S3. A not uncommon mysql node might do 10MB/s. Good luck trying to push that throughput with python or Ruby on a single process through a http response. Much faster I/O exists with sequential HD, SSD, and RAM based I/O. Most networks are built with 1Gbit Ethernet allowing 100MB/s that won't get saturated in the slower languages without going parallel which can increase total system complexity and lines of code.
Note: Also good to do it on cheap, throwaways because this sort of thing burns out the CPU's. Better that box than my main one.