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Virtually zero. One hundred classes that each listen for one of one hundred separate events has the (nearly) the same impact as one class with one hundred listeners in it. The difference is all start-up time when Forge has to do discovery and some bytecode magic to create the hooks, but runtime is going to be the same.

A counter is fine, but an another way of doing it would be to increase a number every tick and then use the modulo operator to compare the current value. So for example, create a ticks field, run ++ticks every tick to increment, then do if (ticks % 10 == 0). It will return true every ten ticks. The modulo operator is basically a division operator that returns the remainder, so comparing it to zero will only return true if the value is cleanly divisible. It won't make a noticeable difference performance-wise, but you may find the code to be a little cleaner. It's just a personal preference thing.

Issue appears to be SecurityCraft...looks like their own VersionChecker utility class is pointing to a non-existent resource during modloading in the client. This appears to make it to where your connection is refused because the server thinks there is a version mismatch between the client and server in that the server just thinks your client doesn't have SecurityCraft. Basically your options are to try and see if SecurityCraft needs to update or if you are using version that is incompatible with your version of Forge, and if that fails either open an issue on the SecurityCraft github or find a SecurityCraft discord./forum where they can provide assistance in making sure your server and client environments are properly set up.

With that hint, you have catapulted me into the depths of knowledge which I needed but did not even know was there. Thank you so much.