The glitch is with some amount of unions, they rerender, causing lag for the client.
(Similar to a topic made a long time ago: Unions Reloading Causing Mega Lag)
This is happening at this place:
(Warning might make inactive due to the place being published under a group game)
This is a video of a Titanic sinking in ROBLOX.
This is about how many parts the ship in the following video had:
Note 3700+ unions, with this many unions, some glitches appear to happen with rendering them.
Info to note: The ship is client side. The server sees an anchored ship.
Even if the ship is not moved it re-renders the unions in what seems to be a certain time interval (no evidence of time interval, just a guess)
A look at what you should be looking for in the video: