How Speed Index is calculated
A testing tool records the page loading as a series of frames, like a filmstrip, and compares each frame with the finished view. From that it works out how complete the visible area was at every moment, then turns the whole curve into a single time. The earlier the screen fills, the lower the number, and lower is better.
Because it reads the whole curve rather than one event, it captures something the single-moment metrics miss. A page that shows its heading, then its text, then its images gets credit for each step. A page that stays blank and then appears all at once gets no credit for the wait, even when both finish together.
Why Speed Index matters
It is the closest a test comes to describing how loading feels. First Contentful Paint tells you when something appeared and Largest Contentful Paint tells you when the main element appeared, but neither describes the stretch in between, which is exactly where an impatient visitor lives.
It is also useful for comparison. Run the same page under the same settings before and after a change and the difference is easy to explain to a client, because you can put two filmstrips side by side and point at them. That argument lands better than a movement in an abstract score.
Where Speed Index goes wrong
It is lab data, produced on one simulated device and connection, so it is not what your visitors experienced and it is not something Google assesses. The graded metrics come from real Chrome users, and Speed Index is not among them. Treat it as a diagnostic, never as a verdict.
It is also easy to compare badly. Change the throttling, the device profile, the test location or even the time of day and the number moves, so results from two different tools tell you nothing when placed next to each other. And it can flatter a page: filling the screen early with one large background image scores well even though the text a visitor actually needs is still missing.
How to act on it
Work on the top of the page first, because the visible area is the only thing the measurement looks at. Get the essential styles in early, let text draw in a fallback font instead of waiting, size and compress the images that appear above the fold, and stop anything further down the page from competing for the connection.
Then look at ordering rather than totals. Speed Index rewards content that arrives progressively, so anything that forces the browser to wait on one large file before drawing at all hurts it more than several smaller requests would. Use the filmstrip to see which frame is still empty and fix that specific thing, rather than optimising the page in general. Where the answer turns out to be structural, it belongs in page speed optimisation rather than in a plugin setting.