We loaded up and tried to continue our 50h+ playthrough this evening - we found the problem to have gotten worse still. We managed to see freezing / 0fps periods in new contexts - summons disappearing for instance, as well as one player trying to level up while the other wasn't. Really bad lag spikes bordering on freezing on the lava filled areas of act 1 etc...
Good spot re the ign article - commented on their appalling qualification of the split screen as mostly okay. Damn they sound like estate agents talking about dumps.