Server Response Time
Server response time, measured as Time to First Byte, is how long the server takes to send the first byte after a request. Anything above 600 milliseconds caps every downstream performance metric no matter how well optimised the front end is.
Why it matters for your rankings
Server response time is the floor beneath all other speed work. If the server takes a second to respond, your Largest Contentful Paint cannot be better than a second regardless of image optimisation.
It also directly limits crawl rate. Google measures how fast your server responds and deliberately slows its crawling when responses are sluggish, which means a slow server reduces how much of your site gets indexed. Common causes are unoptimised database queries, absent caching, overloaded shared hosting and geographic distance between server and visitor. Each has a standard remedy, and the fix usually improves rankings, crawl coverage and conversion simultaneously.
How to check it on your site
Measure TTFB
PageSpeed Insights reports it, and WebPageTest shows it broken down by connection phase.
Check the crawl stats report
Search Console shows average response time as Googlebot experienced it, which is the figure that affects crawl rate.
Add caching
Page caching, object caching and a CDN address the majority of slow response problems.
Test from your audience's location
A server in one continent responding to visitors in another adds latency no code change can remove.