I must admit to using the "cheater line", but I don't think it, in and of itself, slows down play. What slows down play is waiting for another player to play his stroke before beginning to read the green, settle in for your pre-shot routine, etc.
A cheater line in and of itself, does not slow down play.
I use it all the time and don't feel it holds up play at all.
Maybe I'm in the minority, but to me, what slows down play is everything that takes place once another player has played his stroke. If you are reading your putts, taking a look at the contours of the green while the other player is getting ready to play his stroke, it shouldn't take long to play your stroke.
As I said, maybe I'm in the minority, but it doesn't take long to set your ball down on the intended line, using a "cheater line", and playing your stroke. What takes time is reading the greens, from all angles......