Here is what I believe is happening: On my server (vanilla ofc), zombies track down Villagers from far away and calculate the best possible path, while ignoring the player completely. Closed doors don't interrupt this path, so after a while, whole zombies hordes from all around the area gather together on that particular door and keep knocking. This goes on for like forever if a porch is covering them from sunlight.

Issue: CPU usage goes up from an average usage of about 5% to more than 45% for just a single player. After slaughtering all zombies, everything goes back to normal.

I'm no expert but this is probably related to the pathfinding system. Maybe closed doors should interrupt the path, like fence gates do.