As long as a villager is going to or from the nether, chunk updates are frozen. As soon as the villager finishes going to the other dimension, all chunk updates that had been suppressed happen at the same time.

I had a video, but it was too large, so I'll just explain what happened:

I placed the villager in the portal, attempted to spawn more villagers with spawn eggs (Nothing happens), and attempt to power a piston with a redstone block (No activation). Upon the villager going into the nether, all the other villagers I spawned appear at once, and the piston extends