What is server response time?

Server response time is how long the server takes to start sending the page after the browser asks for it, before any rendering happens:

• It is a floor on everything else - no amount of front-end optimisation recovers time already spent waiting.
• Common causes - shared hosting under load, an unoptimised database, no caching, or a server far from your visitors.
• Caching is the usual fix - serving a prepared page instead of building it on every request.

Google reports this in PageSpeed Insights as the initial server response time; it is the first number worth reading.