When scaling WordPress sites, one of the first things people try is adding more hardware. Unless this is a proven bottleneck in the stack, this added hardware capacity is often left idle only to be used by various buffers and caches later.
So how do you do capacity planning for large WordPress sites? At Scale Dynamix, we like to think of it in terms of latency and throughput.
Roughly speaking, latency is how fast the site loads for one visitor. Throughput is how many visitors can access the same site at the same time. If latency is too high, adding more hardware makes little difference.
Imagine you are going on a road trip. To avoid traffic, you leave at 2 am and have all six lanes of the interstate to yourself.
You are driving a Toyota Yaris! Would adding another lane on the freeway make you go faster? Of course not.
Similarly, if WordPress is slow for one visitor while most of the CPU is idle, adding more hardware won’t make much difference.
This is why the right starting point when scaling WordPress is latency reduction. Making sure the site loads as fast as it can for one visitor helps when you are ready to open the floodgates and start serving thousands of visitors per minute.
Having said this, I would choose an empty freeway and a Yaris over a congested road and a BMW any day.