The reason people (everyone) are having FPS issues is because of the Bot Nuke and how Jagex reworked the coding of the game. I'm not going to pretend I know exactly how it all works, but essentially they changed the way you receive updates from the game area and how you interact with it, in order to stop injection bots from working. In turn the game became less optimized and now takes longer to process which results in lower frames per second.
They have stated that they are planning a second bot nuke, which they are calling "Optimus," that will be released in to the game sometime in April or May. Optimus has two main objectives, stopping the current color picker bots, and optimizing the game to run better while still preventing bots from working.
As for now there is nothing you can really do except turn down your setting and know that you aren't alone
