I personally haven't experienced any issues with data on my device, seems like BitKong is mostly cached, so doesn't need to be loaded from scratch every time.
Ditto on that, I haven't noticed any performance issues or data unusual amounts of data being used.
Anybody seeing this might have an issue with their browser cache, e.g. caching disabled? Playing on incognito mode can cause this, since the site needs to be reloaded every time and no assets are saved.
Luckily they have their own Android app for anybody concerned with data usage:
https://bitkong.com/en/android