I want to criticize the choice of headline here, which was no doubt the point of having it. It's not about PHP. PHP runs fine on Pis. Not that I would ever advocate running a website with serious traffic on that device, but it could very well serve a 50-100 dynamic requests per second (I just tested this with my Pi B running NginX and my custom home automation software which is web-based, the baseline is 250 static small file requests per second, so not a great starting point for publicly hosting things).

Instead, this should be a critique of Wordpress, which is like almost any other successful software immensely bloated. There is nothing inherent in PHP promoting this verbosity. And even if you are dealing with large code bases, PHP's request-based execution model gives you good tools to just execute the codepaths you need. But in practice, little consideration goes into minimizing execution bloat and overhead. Of course there are reasons why WP is bloated, some of them bad, some good, some subject to debate. Caching and avoiding dynamic code execution will be a big part of optimizing any Wordpress install for the foreseeable future.

We're moving towards more bloat, not less. The best practices of many programming environments actively encourage us to disregard cycles and latencies. This happens in PHP, in Node, even in C++ frameworks.