I have timed myself separately when doing SAT practice. I would time myself on a separate stopwatch, and I would practice thoroughly whether it be math or reading and writing. I think adding a stopwatch could be a great addition to see how long it took you to thoroughly go through the practice. Though not everyone will want to time themselves, the stopwatch could be an optional feature right before starting the practice. For example, let's say you clicked on Practice for Reading: Science. There would be a separate page whether or not you wanted to initiate the stopwatch, and whether or not you wanted to see it or hide it. During the practice, you could click on the stopwatch to see it or hide it.
Also, under the Review tab in Official SAT practice, with each practice, you could see the time it took you to go through the practice if you initiated the stopwatch.