iPhone 5s wi-fi unusable

Wi-fi on my iPhone 5s is virtually unusable. Sometimes it won't accept a password even though the pass is definitely correct. Sometimes it will accept the password, but lose connection within minutes. Even if wi-fi works, it's too slow to be usable. I've tried a speed test using the Ookla speedtest app, and the test usually can't even be completed. If it is completed, the speed is always 1mbps or less. If I turn off wi-fi and do the same speed test using cellular data, it's around 15mbps.

I'm on a Google Fiber internet connection. My 2012 Macbook Pro sitting 2 inches away from my iPhone and connected to wifi ran the speedtest at 542.64mbps/download 932.5mbps/upload just moments after the iPhone couldn't even run the test. My iPad mini isn't great, but better than the iPhone. Sitting directly next to the iPhone and Macbook, the iPad mini gives me 4.29mbps/down and 1.19/up...not great, but at least usable. These are all typical results...Macbook at 500mbps+, iPad around 5mbps on a good day, iPhone 1mbps or less in the rare event it even connects.

What's the deal? I reset network settings. It fixed nothing. I turned off wi-fi networking under the iPhone privacy menu (because some random internet article said it would help). It fixed nothing.

My cellular data plan is getting eaten up because I can't use wifi at home. I now have to turn off cellular data every time I come home so I don't burn through half my data plan in three days.

Help? Thanks.