Sadly, this been an issue since day one when TW was introduced. The game never has rendered the graphics or music to disabled because the coding wasn't done right. The proof is obvious the fps ratio drop is not consistent to what it should be if an image is disabled. Tell you the truth it's rather not even noticeable performance with it on than it is off.
They really need to update their engine codes, but do not even bother. Sorry to say this, even if sounds tasteless, but games made in the Far East are just this terrible put-togethers which sees tiny improvements. They are always late to the dance so to speak.
The only solution is what I've done: uninstall and play a new game. Personally, these same problems with this X-Legend game is noticeable in Aura Kingdom.
During my years of MMO I can say this quite comfortably: It's not worth playing a F2P made in Asia with a high end PC. You'll come to learn that component deterioration is more quickly because a PC cannot force a software above it's max, but the vice versa is true.
So if you having problems with graphics with this game I strongly considered a custom built cheap midrange or just find a new game because your solutions and observations are correct but falls on deaf ears. X-Legend just does not care until it becomes too serious.
Ever server maintenance Aeria Games leaves at the bottom of the notes about the latency issues. Ironically, it's been copied/paste since day one like a permanent footnote on how much the developer cares.
By the way, there are other skill spams from multiple users that have the same dilemma like Black explosives. This problem is not a new issue.