Why does VR on Android run noticeably slower?

I have a VR app meant for iOS and Android devices. When I run it on iOS, it runs smoothly without a single hiccup at 60 FPS (capped at 60 by code). If I run that same app on an android device, it runs at 26 FPS average, but can drop as low as 18 FPS. For VR, that is just unplayable, unacceptable.

The devices used for testing, on iOS, is the iPhone 7. On android, I used the Nexus 6P and the Galaxy S4 (so it's a 2+ year difference between android devices), and both run exactly the same, although the 6P has considerably better specs.

How is this possible? if an old device, and a newer more powerful one run the game exactly the same (android), and iOS runs it more than twice faster, it makes me think that Android and Unity aren't playing along.

Are there any settings I should change in the graphics, quality, player, etc. to get the app working equally in both OS's?

Thanks a bunch for any insight into this!