I should have had the best opportunity to show poor wifi performance with a ASUS repeater on 5Ghz passing through the main Apple router, but after several tests on Ookla's speediest, I see a solid 50mbps download, but a bit more variance in the upload...should be solid 5 and I see it bouncing all over the place. The actual performance is excellent off my phone. Be aware that the test is impacted by the location, one location could have poor upload or could be hammered. I'll see if I can post my result.
Also, considering it drop off, I would see a performance hit as I have very little cell reception, not enough for any internet connection, cell only.
I should also say that the router is configured like this:
Cable modem--LAN--> Apple Extreme -- 2GHz N-->Asus -- 5GHz AC --> iphone