Server lags in the first few minutes of the game

Hello, I have noticed that on my map in the first minutes of the game occurs something like in the video below

Does anyone know what is going on? The only device (I think can only cause this) used are Sentry and Guard, beside cinematic sequence (but Sentry&Guards are spawned only if player is teleported into box)
I was thinking about world partition(CellSize12800, Loading Range: 4000) or something but I dont think it matters. What can cause this?
~if you are afk, or even moving on spawn area for like 1 minute this is never happening so I dont think it is related with World partition

The map is laid out like this(idk if it matters):

image

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