Why does my iPhone 6 drain my wifi?

11mbps does not meet current FCC definitions for broadband.

Anyways devices connected to your network should not affect internet performance for other devices. Unless those devices are actively connecting to an outside source. Say the iPhone is running updates or you are allowing apps to run in the background.

If network performance isn't up to snuff you can setup a new router. You can get a nice enough TP-Link 802.11n router for $20 to $25.

Also 11mbps sounds pretty slow for cable internet. Do you have an old modem? You can buy your own DOCSIS 3.0 compliant modem for about $60. If you are paying a rental fee it doesn't take long for it to pay for itself. Just be sure it is compatible with your provider. They should have a site that lists compatible models.

Sometimes providers don't send out new modems when they updated their lines for DOCSIS 3.0. So many customers still have 2.0 modems. I more than doubled my speed when I bought my modem. Plus I save $7 a month. So far that is close to $200 saved after factoring in purchase price.

iPhone 6 does have some battery issues