We'd probably have to know your phone's model or at least how much RAM it has, your Android version, and whether you're running the browser version, the PC version via JoiPlay, or the APK version, before we could diagnose.
Meantime, if for example you're using the APK, you could try downloading the PC version and using JoiPlay to see if that works better. If it does, you'd also have the advantage of having access to the PC keyboard shortcuts (that's part of JoiPlay's interface) that are typically inaccessible on APK builds.