I work for an AdTech company. One of our selling points is that we track much time users actively spent on our ads (= ad is in viewport + tab is active + user has moved the mouse or scrolled in the past 20 seconds). If we consistently see a source of traffic spending < 30 secs, then we manually start digging into the logs.

If you're a brand/marketer, I recommend two easy things to increase your ROI: (1) Make sure you are not being charged for GoogleBot traffic. Seriously, for frequently updating sites, self-advertising GoogleBot can account for up to 10% of all impressions. Ad Networks can easily filter that traffic out. (2) If you're paying per impression, work with partners that charge on viewable CPM.