@Drew – I actually have a lot of detailed benchmarks available over on the cluster’s GitHub project Wiki: https://github.com/geerlingguy/raspberry-pi-dramble/wiki/Dramble-D8-Benc…
The Pi 2 is respectable compared to a low-end PC or low-end cloud server; it is generally 20-30% slower in my testing than a micro AWS instance or a Digital Ocean droplet, but the performance difference depends on a lot of different things, especially what kind of application you run on it.
Regarding clustering in general: there’s always a tradeoff to be made when clustering; you can aim for better performance and better availability, but if you want both, that requires many more servers. In this cluster, I have 6, with a load balancer, four webservers, and a database server. If the database fails, then everything is offline.
You can also add a reverse proxy so things are cached and you protect your ‘backend’ servers more, and you can set up the database in a master-slave configuration so you can fail over if the master goes down.
Once you get into clustering, the rabbit hole is very deep 🙂