Timed out problem

The main problem is the loading time of the addons. I would try to reduce the amount of content the player has to load.

Another thing you can do is increasing the timeout time. You can do this by changing the convar sv_timeout. According to the old gmod wiki the default value is 65. Try to increase the value until the problem is fixed.

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I think it depends on when the player gets the time out. If it's at "Workshop complete", it's because of the amount of addons they have to load.

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server.cfg

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What do you mean by I've even gone to the map as an entity and which value did you assign to sv_timeout

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Has the time between join and timeout increased?

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most likely tables being stored and filled up with data, at a certain point it just becomes too much for the clients to handle i can only assume

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Something I don't really recomment but maybe try to set the value to 300 or bigger. This means, that players will time out after 5 minutes. If they still time out then it's something with the client itself.

1. Do you have sufficient network capacity to handle hosting a dedicated server?
2. If you are using a server host, check what the speeds are. A minimum of 100 MBPS is recommended, depending on the number of slots you have.
3. If you are not using a server host and are hosting yourself, that may be your problem. Private at-home Internet is often not meant to host dedicated servers and doing so is often a violation of the ISP terms (check with your ISP).
4. You said that your server is not overloaded. Have you been checking both CPU performance and RAM usage? If you have it virtualized, have you allocated sufficient CPU and RAM to the server?
5. Look through your addons and see if you have any that could be very intense network-wise. Maybe use a third-party addon to see how each addon is impacting your server's performance.
6. What are your dedicated server network settings set to? These need to be optimized based off of your available hardware, slots, and expected performance.
7. sv_maxupdaterate
8. sv_maxcmdrate
9. sv_mincmdrate
10. sv_minrate
11. sv_maxrate
12. And more...
13. Steam has an official help article on optimizing dedicated servers. Maybe give that a read. https://support.steampowered.com/kb_article.php?ref=5386-HMJI-5162

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