Speed Index: Speed Index measures how quickly all above-the-fold content is painted on screen. The earlier the pixels are painted, the better you score on metric. Users want an experience where most of the content is shown on the screen during the first few moments of initiating the page load. Loading more content earlier makes your end user feel like the website is loading quickly, which contributes to a positive user experience. Therefore, the lower your Speed Index, the better.
Time to Interactive: defined as the first point at which everything is loaded such that the page will quickly respond to any user input throughout the page. Note: this metric is currently in beta, which means that the underlying definition of this metric is in progress.