'Feature not available because it will slow down the device'

This error normally shows when a device:

• needs to be updated;
• does not have enough room;
• is too old; or
• when it has too many things running in the background.

Changing one of these may help you to get started with the playtest. 

If that doesn't work, do you have another device that you could use? You can take a look at a list of devices you can playtest on - What Devices Can I Playtest On?