It's destroyed my iPhone 6. The iPad Air 1 doesn't stand a chance here.
I just spent a couple of hours nuking and paving my iPhone 6. It was getting a bit laggy, but it definitely runs much faster now after wiping, reinstalling the latest iOS 11 firmware/iOS and reinstalling all my apps (not restoring from a backup). I had not done this since getting my first iPhone 6 years ago. What a difference.
I simply connected the phone to my Mac by USB, started iTunes (latest version), clicked "Restore iPhone..." in the iTunes window, and then when it got to that point I told it to set up as new, not restore from a backup. "Erase settings" and "Erase all contents and settings" in the phone's Settings app won’t replace the operating system; "restore and setup as new" in iTunes will.
https://support.apple.com/en-us/HT201252 describes the procedure. A factory restore erases the information and settings on your iPhone, iPad, or iPod and installs the latest version of iOS software.
I am not recommending this procedure, just reporting that it worked great for me. The procedure results in loss of some data, like game settings and I lost my 1SE (one second everyday) video, but not great loss and I could have saved the video. Everything else is in the cloud. Also had to wipe my Apple Watch and re-pair it. So one needs to read up a bit before doing it. But from front to back it only took me a couple of hours while I did other work at my desk.