The Speed Index score is a metric and an abstract number rather than a point in time. It does not have a unit such as milliseconds as it is not a time measurement, but a score on how the pixels change or rendered based on the filmstrip intervals. It uses the same logic as webpagetest.org but relies heavily on using the images provided by Catchpoint filmstrips. If the filmstrips are set to capture at a slower rate, the interval in-between images increases and the score can fluctuate as a result of large changes in the page within that gap of time. This metric also depends on a whole range of factors such as browser and viewport size.
Enable Capture Filmstrips is a requirement to gather score data for this metric.
For a deeper understanding, you can visit our blog post here.