The funny thing is that the only place I have ever heard of that is on Mindcrack videos.
So far, I didn't notice any more lag from these tanks than from other multi blocks.
The fact that they can be bigger, and composed of nearly any blocks might make them take a bit more resources, but nothing as major as they claim.
If anyone has any proof of the contrary, then by all mean tell us, because the Mindcrack guys tend to point quite randomly at things for lag, and completely overlook others (such as Guude having more than 60 stacks of cobble in a single iron pipe, going back and forth and causing explosions).
Usually, server lag is caused mainly by entities and block/light updates. Fast timers and anything causing light to update are usually a good place to start hunting. Too many chunks force loaded won't help either.
Client lag can be caused by many things, including entities, graphical updates, and blocks with custom render in great numbers.