I don't think they are mining but the high load is because of too many ads as this happen with me whenever I use a site that have a high number of ads.
Given that they're using every way they can to gain profits from their traffic (Ad views, surveys, referral links... etc) It's obvious that they would consider mining too, especially when you know that other similar sites offer mining as one of the ways to get credit. popspins.com for example. same business model. another website to skip.