Load Testing is multi-disciplinary. It's just one aspect of the broader concept of scalability of any software application. It requires broad experience to get it right. Done right it will net out several projects that will improve the overall scalability of the site / product in question in several dimensions best illustrated by the AKF partners scale cube.
There are many factors that impact load testing on a mature application...
- server configuration
- data center setup
- single points of failure
- dev/stage/prod environment separation
- database... oh the databases...
- the CDN
- the graphics coming out of the design team
- Mobile network speeds and latency
- Inefficient Code / Bugs that only come out under high load or concurrency
- the deployment process
- missing documentation
- botched parameters on service / micro-service configs
- and this is the short list...
Meaningful load testing considers all these things and more. Done right, it shines a light on people and processes that are sometimes surprising. It can be painful because it is DESIGNED to show the flaws and break people's hard work. As a result successfully spawning projects from load testing results takes a bit of finesse and often executive support. It's very much about overall operations and site building process. Knowing the ins and outs of the entire product development lifecycle is a bonus. Serious load testing falls into the CTO / SR. Exec Tech role. It is not particularly fun work most of the time. This is why everyone puts it off for as long as possible.