It definitely wasn't slow. I've had it in this building since I first made it and only today did it start damaging stuff. At the rate it was damaging the building 2x20 refined eitr would have had the building destroyed.

I guess I'll look to move it further away and encase it in black marble. Thanks for the info about black marble.

Edit: I guess for whatever reason I've been lucky that it wasn't doing damage from day 1. Or unlucky depending on how you want to look at it.