How the number displayed in spotlight for max IO wait is gathered (4314123)

We run fn_virtualfilestats on a regular schedule, which it runs the QS_IOStall.sql found in the Agent\conf\Package\sqlserver_spotlight\2005 folder

It either 15 seconds if the Homepage is open or 5 minutes otherwise.
We look at the IOStall column for 2 consecutive executions and take the difference between the 2.
This gives us how much time was spent waiting on that file over that period of time. So for instance we might get 100ms over the 15 seconds between 2 collections.
We also look at the number of io operations over the same period and calculate the difference between them.

So now we have the number of IOs in a period and how much time we spent waiting.
By dividing the time by the number of IOs we get the average amount of time per IO.