recently installed xcode 9.4.1 on my high sierra
and still same problem, simulator painfully slow,,,,,
thou se, 6, 6s, 6+, 6s+, starts within 3-4 min, but other then that, (7,7+,8,+8,x) never starts,
waited even 1/2 hr, still spinning wheel a nd nothing happens....
and often get springboard...