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I know this has been denied many times, usually in the context of that it's hacks and/or unfair advantage, but I have a few thoughts in the crystal optimisers favor., at least, in my opinion.
***Note:*Hacks, in my opinion are a mod, plugin etc that does things for you, like aiming. Unfair advantage in my opinion is a source of advantage you get not from your pvp skills, like X-Ray
First, what does a Crystal Optimiser do? It allows you to send more packets (the way your pc and the server communicate) per second with the data that says you placed/broke a end crystal. The packets are triggered by the player, meaning the player has to right/left click on the correct conditions to send one packet saying that you placed/broke a end crystal. The mod doesn't right click for you, so it's not a hack.
Second, I wouldn't say that Crystal Optimiser's are unfair advantage, at least, in the context that resource packs and fps boosters aren't.
Because if two players veresed each other, with the same skill, one with without a fps booster and is at 10 fps, and his opponent has a fps booster and is at 100 fps, then his opponent would easily win. His opponent won because he got advantage from the fps booster, not his pvp skills.
Same thing with resource packs. Vanilla resource packs are extremely obstructive for cpvp, so if a player with the vanilla resource pack versed someone with a proper cpvp pack, then he would have a hard time winning. Once again, he got the advantage from the resource pack, not his pvp skills.
If you still think it's hacks and/or unfair advantage, you could make it conditional, for example it would turn on only on the servers that allow it, and if you have high ping. Additionaly, you could limit how much you can crystal per second.
With those conditions, I think it would balance out how much advantage you get from ping difference, a high ping player with the optimiser would stand a better chance against a low ping player without the optimiser.
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