I have spent a couple hours searching this as I had come into a similar issue. I have found the problem, but as I have no result for a solution. The problem is the doors, but it also the villagers. The way Tango set up the iron foundry, the 2 sets of villagers are "detectors" the reason for the lag is because ~40 villagers are connecting to 44 villages. The villagers connect to 'village 1' first, next the SAME villagers connect to 'village 2' ... and so on. By the end all 44 villages have ~20 villagers in them. Some how when the a single villager connects to more than one village it somehow is making intensive activity happening server-side. (On my computer its roughly 5 villages per one villager before I start to feel the lag). Hopefully I've explained this decently. I don't know anything for-sure but from a couple of hours of testing I think I have a pretty good idea.
I have spent a couple hours searching this as I had come into a similar issue. I have found the problem, but as I have no result for a solution. The problem is the doors, but it also the villagers. The way Tango set up the iron foundry, the 2 sets of villagers are "detectors" the reason for the lag is because ~40 villagers are connecting to 44 villages. The villagers connect to 'village 1' first, next the SAME villagers connect to 'village 2' ... and so on. By the end all 44 villages have ~20 villagers in them. Some how when the a single villager connects to more than one village it somehow is making intensive activity happening server-side. (On my computer its roughly 5 villages per one villager before I start to feel the lag). Hopefully I've explained this decently. I don't know anything for-sure but from a couple of hours of testing I think I have a pretty good idea.