Ideally the browser caches the assets when it downloads them so that future downloads will be instant. If downloading is not fast enough the first time, try doing it a second time to see if caching is working. If it is, the download portion of startup should be instantaneous.
There are two types of caching. The core game is cached by the browser. We have little control over it. Generally the browser will try to hold on to often-used content so Imagine Math Facts is cached effectively. There are some practices that can set a browser to automatically clear its cache constantly.
Once you load the main game, each individual game is cached in a different location unless you’re using devices with an ARM processor. Some schools configurations may also alter caching locations.