I guess you mean a 2013 Mac Pro, there is no 2015 model. The Mac Pro didn't get any update for almost 3 years.
Anyway, 8 seconds is not right, by considering the Mac Pro's SSD speed. Even though after a cood boot, the system preference should easily pop up in 1-2 seconds. And for me, my system preference come up in less than 0.5 seconds.
I suspect you hit some bug, but don't know what it is.
e.g. All my apps loads fast, even photoshop, FCPX, they shows up quick. But it cost 8-10 seconds for Messages to open. I don't know why, only this one, and it wasn't like that few months ago, obviously something wrong. However, since I almost never close it, so it doesn't really a matter to me.