According to this https://sites.google.com/a/webpagetest.org/docs/using-webpagetest/metrics/speed-index "The Speed Index is the average time at which visible parts of the page are displayed. It is expressed in milliseconds and dependent on size of the view port." Reading the explanation, I doubted that.
Essentially you're making an integral of the % visually incomplete. That means taking the area under the curve % v Time. Which to me suggest the metric has something like units of %/s.
Any thoughts on this?