The issue is that its hard to track that and its easily messed up. First you have surveys. You can't use how long the HIT tab is focused for these because you are working in another tab. Then you have batches, where the time to complete the HIT is generally how long that window was focused for. Its a mess and while it can be somewhat completed it isn't all that accurate. You could make it so the timer keep running for the HIT unless another HIT was viewed, but then you could open a bunch of surveys from your queue at once, look at a few and mess the tracking up there.