Today, Microsoft revealed that Xbox Cloud Gaming on iOS has received major performance improvements. Beforehand, the service worked great on Android devices, but suffered from more input lag and severe image quality problems on iOS. It's unclear if this was due to the fact that you have to log onto a website to access Xbox Cloud Gaming on iOS.
I tested out the update by playing The Elder Scrolls V: Skyrim - Anniversary Edition and a few other games, and can say that there is a noticeable performance boost. The input lag is definitely reduced and the image didn't freeze during my 30 minutes of gaming time. However, there were a lot of video artifacts that you don't see on Android.
Hopefully, the team will continue to improve the experience on iOS devices in the months and years to come. Having a dedicated app would've been the ideal solution, but until Apple reverses its policy on game streaming services, we'll have to use the website.