To get help from Jagex use the Support link on the top edge of this page. However, for some of Jagex's support articles, if no solution is found, they refer you to the community led forums. Some other support articles provide a way to contact a J-mod for support. Perhaps the support article titled Improving RuneScape's Performance loosely applies to your issue. If you don't find a solution there is a Contact Us link at the bottom of the article.
OSRS (with the official client) is heavily single-threaded, and there isn't much that can be done to fix that. It eating a single core (whether in-game or at the login screen) is normal.
Using a 3rd-party client can partially help reduce CPU load by using a GPU renderer, but that requires you to trust such a client, and this still doesn't totally fix OSRS primarily using a single CPU core.
Options include dealing with it unfortunately, and/or asking Jagex to optimize their game.
I have 2 Opteron 4386's; 16 cores for nice multi-threading, but terrible single-threaded performance. OSRS at its best gets to the low 30s with FPS, averages low 20s, and in places like the great library, I get single-digits. I tried Java versions 8 through 12, with many different flags; I've found nothing that fixes this single-threaded mess, and nowadays I just play the main RS game since it's actually optimized enough to use multiple CPU cores.