Gmail.app for iOS is incredibly sluggish (on my 5S) while Android's version on my Moto G is pretty snappy, so you're not experiencing the same thing at all.
I run both Mail.app and Gmail.app on my iPhone because I'll often get the push notification for Gmail.app, tap the notification to open the email, get frustrated and tired waiting for it to load and just go manually refresh Mail.app and see it almost instantly.
The Gmail app has always felt like a web app and I much prefer Mail.app
Admittedly I'm an Android user with limit experience with stock iOS mail app. Is the iOS mail app really that much better than the Gmail app?