What TTFB measures
Time to first byte is how long a visitor waits, after asking for a page, before the first byte of it arrives. Nothing can render before that, so it sets the floor for everything else - LCP included. Google's guidance is 0.8 seconds or less as good and over 1.8 seconds as poor for real visitors, measured from the start of navigation.
Why test from many cities
A single test only tells you how fast your server is from wherever the tester sits. A site hosted in one country can be quick there and slow on the other side of the world. This test sends real requests from probes in up to ten cities at once - London to Sydney - through Globalping, jsDelivr's open network of measurement probes, and shows DNS lookup, connection, TLS handshake and server time separately for each.
Reading the result
Short DNS, connect and TLS times with a long TTFB mean the server itself is slow - a database query, an uncached page, a cold start. Long times only from distant cities mean the page is coming from one far-away server: put a CDN in front, and let it cache HTML, not just images. The cache column shows what the CDN did: HIT is served from the edge, MISS went to your server. Run the test twice - the first request in a region often fills the cache.
For a single address with a breakdown of page weight, compression and render-blocking files, use the website speed test; for whether a page passes Google's thresholds, the Core Web Vitals checker.
