The Site Performance Index can help to quickly gauge how well a website is responding for any visitors.
How is the Site Performance Index calculated in App Synthetic Monitor ?
The Site Performance Index is calculated as the average of connect time + download time for all successful checks in the given period, plus the SPI Penalty, which is calculated as a penalty of 10000ms for each unsuccessful check, divided by the total number of checks.
With lower timeouts there is a higher chance of unsuccessful checks, therefore the SPI penalty could he higher than with higher timeouts. The same goes for the monitoring interval - the SPI penalty will inherently be higher for monitors with a higher monitoring interval.