Full full full dead
Let's look at it another way: How does avoidance compare to effective health for maintaining healer mana? This will be in the context of heroics or level instances. Everyone talks about raids, so let's try something new and discuss something that isn't worth the bother.
If the tank avoids, you don't need to heal. Assuming the tank won't get two-shot, you can heal reactively. More avoidance means you don't have to heal as often.
The tank takes more hits, but you can let him drop more before healing. This means you can heal in bursts rather than now and then.
A useful post would bring in math, but that's far too complex for writing while I wait for glyphs to be posted. Instead let's just think really, really hard. Is the healer going to maintain mana better with less healing, but potentially not getting out of the 5 second rule? That's the avoidance tank. The EH tank will need more healing overall, but it can be more like heal heal heal heal wait..... heal heal heal heal wait.....
This is obviously irrelevant to a healer based on mana/5 or crit, such as a paladin or shaman. But for priests and druids? It could make a difference.
But let's get back to the point: Do bad tanks cause lag?
What is lag? Lag is what it is when a healer doesn't heal you soon enough and you die. An undergeared tank will reduce the reaction time, making it more likely that the healer will lag. By lag I of course mean 'lag'. Or do I?
Could it be that the lag complaints in Dalaran are not due to tons of people in a small space, or low RAM or slow hard drives, but instead could it be caused by a crowd of bad tanks?