On a Mac that doesn't have a built-in display (like a Mac Mini or Mac Pro) you should connect an external display when you're using VoiceOver. When the built-in screen reader is turned on, some apps might become slow or unresponsive if there's no display connected.
If you need to use VoiceOver without a connected display, you can connect an EDID adapter to the HDMI, DVI, or Mini DisplayPort connector on your Mac. EDID adapters are different from video adapters – they aren't used to connect a TV or monitor to your computer. Instead, EDID adapters send a signal to the video port so that your Mac acts as if a display is connected.