I started a skirmish game, kicked the computers butt and then thought what else I could do.
Started mining tiberium - big time. Of course there was the problem of storage so I built a few storage silos. At just under 150,000 I got sick of building silos so I started manufacturing heavy tanks, this wasnt good enough with just two factories, so I built five more.
Some time wasted later and I had completed the task, very little tiberium crystal on the map and my silos were empty
Then I noticed how many tanks I had made, dozens of them, possibly a couple of hundred. Yet the interface was still fast and didnt skip at all - even when moving huge groups
Of course that might change when I go over to fighting more than one opponent, upping the capability of the computer controlled players might increase the issue too.