I've read somewhere that unfinished multiblock structures lag a lot since they constantly check if they are finished yet. I think it was about Railcraft's stuff mostly but is it true for unfinished big reactors too?

Sorry if it's the wrong place to ask, I was trying to find forum or something, with no luck.

If I recall correctly the multiblock only does a check on block update which means as soon as you place or break an adjacent block the multiblock will be recalculated.