Redis vs Memcached vs file_get_contents
I read articles about web performance and scaling almost every day, and when it comes to caching, the vast majority of them promote tools like Redis and Memcached, which are really fast, in-memory key-value stores.
Their performance metrics, the requests per second, how easy it is to scale them and all their great features, will often overshadow the fact that these are services designed to run on remote servers.
In a single-server setup they will very likely hurt your performance, compared to simpler tools like reading from and writing to the local filesystem, with enough RAM to utilize for the Linux kernel page cache of course. RAM is still faster than SSD or NVMe, nothing changed about that.