In an optimized system the bottleneck will be I/O. However elements like serialization, many function calls, and extraneous memory copies chips away at performa

In an optimized system the bottleneck will be I/O. However elements like serialization, many function calls, and extraneous memory copies chips away at performance to the point it matters and its difficult to address these problems directly in languages outside of C/C++. 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.