Are we talking unusably slow, or just noticeably slower than all other browsers? For example, what does this link report for FPS.
If it’s the former, you may be experiencing software rendering caused by IE’s aggressive and large hardware black list. If you look in the IE dev console when running Cesium, do you see any messages about software mode or rendering? The best solution here is to make sure your video drivers are up to date and that Windows is fully patched, even then IE 11 might refuse to use hardware accelerated graphics for your card (what graphics card is it, by the way?)
While there’s no magic “make faster” setting you can enable (if there was, we would already be doing it for you) you can try and make a trade off between quality and performance by rendering at a lower resolution in IE compared to other browsers. You do this by setting the viewer.resolutionScale property in IE only, for example, try it with 0.5 and see if it affects my first link.