A slow device is a device that cannot handle the full speed of a song in the game. Depends on how powerful the device is as well.
On iOS, the device can sometimes get slowed down when Screen recording is on. This is an absolutely rare glitch.
On Android, you can easily slow down the game by turning on "Show screen updates" in Developer Options, or by turning on Power saving on a Samsung device.
There are also certain songs that can get slowed down on iOS, if it's an iPad 2 or iPad Mini (1st Gen), or iPhone 4s. Turkish March mostly does that, probably because the device does not have enough RAM to handle the speed or length of the song. Also, Turkish March is known to use more and more RAM than any other song when it gets sped up. Humoresque No. 7 also does that, but this is rarer.