Using "online-mode=true" when I try to execute any command that needs to fetch gameprofile of player that weren't on the server (like /ban /pardon /whitelist add /whitelist remove /op /deop) then it creates lag on the server. This lag sometimes can take even 10 seconds. The player must not be fetched nor cached by the server for lag to happen.

I think it might be caused because that request to Mojang API happens on the main server thread.