I've been here with a friend of mine before due to the same issue. PowerRanks permission lookup function seems to still be kinda slow. This can cause huge lag spikes when spamming tab completions because WorldEdit abuses this function by checking any tab completion for permissions many times. This behaviour of WorldEdit in combination with the slow permission lookup of PowerRanks allows easily timing out the server which causes it to crash with malicious modifications.

Maybe add a small caching feature that caches the lookup for 50-150 milliseconds? That would probably already speed it up enough. Another implementation of that cache could also include indefinitely caching results until some state changes like adding/removing ranks or permissions.