That isn't going to catch sites that load reasonably but behave slowly afterwards (eating CPU time so potentially slowing other apps down too).
There are a couple of sites I visit that do this. If I leave a few tabs of imgur open in Chrome they'll consume a pile of CPU time (this doesn't always happen, so perhaps it depends which ads are on the page?). IFLS pages eat CPU and memory too (again, almost certainly due to the ad content). In both cases it can be noticeable on a powerful desktop machine, so it must be a significant drain on low powered laptops and such.
There are a couple of sites I visit that do this. If I leave a few tabs of imgur open in Chrome they'll consume a pile of CPU time (this doesn't always happen, so perhaps it depends which ads are on the page?). IFLS pages eat CPU and memory too (again, almost certainly due to the ad content). In both cases it can be noticeable on a powerful desktop machine, so it must be a significant drain on low powered laptops and such.