Its almost impossible to join servers with low end PCs in ArmA 3 because of signature check timeout. You join the game, select slot, remain in loading for few minutes and get kicked off the server right away after you just entered the game. Here is what server_console.log says, it took me more than 2 minutes to load and I got instantly kicked when loading was finished.

6:36:27 Player Sa-Matra connecting.
6:36:28 Player Sa-Matra connected (id=uid).
6:38:46 Player Sa-Matra: Signature check timed out
6:38:46 Player Sa-Matra disconnected.

This problem also was actual in ArmA 2 when you tried to join server that has been running for long time and has lots of objects\vehicles on the map and loading takes quite a while.

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Another detail that might be related to the problem, server runs addon which clients don't need to have, maybe it somehow triggers heavy signatures check that lasts several minutes for all players.

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This also happens in lobby, you join the server and few second later lobby goes into long several minutes freeze and then its gamble either you will be kicked for signature check failure or not.