OK, not too laborious. But the fact that the posts don't show up in the index still makes for a clumsy transition.
But I think that it probably is a good idea to do that kind of thing when a thread deserves to be carried on in a way that isn't excluding people who can't load long ones. Typically what happens when a thread is renewed is that the points which were made in the last few posts get overlooked, since there are people who won't have seen them because of the loading problem.
The convention seems to be that the thread renewal point is when it gets over 100. I think it probably should be a bit lower, at a level where everybody can still open up the link to the previous thread without a problem. Maybe around 80?