The IO over head (in reading a file and sending the data over network) is huge, it overshadows the performance gain obtained by using C++. If a server does huge number crunching then using C++ is necessary. Transcoding an audio or video, image processing, natural language processing etc require huge computation and this kind of work load is where C++ performance is required. Based on the need the programming language is decided.
Zero copy data transfer from hard disk to network card, async data read/write, etc can be difficult to implement in some high level language. That is why web servers like apache, nginx, node are written in C or C++ as they need multi-threading, async IO, fork / exec and other system call access to make serving data fast.
Example: Google page ranking algorithm uses matrix operation, svd etc. It requires huge number crunching so it uses C++ and Fortran libraries that does number crunching. To serve search result C++ is not necessary at all.
When a video is viewed in youtube the server spends most of the time on reading the file and transferring it via network. But when a video is uploaded the youtube server starts encoding the video in different format (mp4, webm etc), different resolutions. This involves huge number crunching, for this youtube uses ffmpeg which is written in C++.