From the same Settings page where you can enable Narrator:
To turn Narrator off, press and hold the Volume Up button, then press the Start button.
For more information, see Using Narrator on your phone.
You can control the speed of the narrator using a slidebar.
Go to Ease of Access -> Narrator.
You could see a slider under the speed option.
Drag the slider to increase or decrease the speed of the narrator.
If you go to Settings => Easy of Access => Font Size and increase the font size there, built-in Internet Explorer will respect this and increase font size of web pages. It works this way on Windows Phone 8.1 for my Lumia 920.