Greetings Maha,i have analyzed youe system file and just replaced a white source led with lower power same led.you can set the value as low as -130 dB in power tab.Then it will calculate in a fraction of second.This is happened due to reason that for higher power smf calculation takes too much time and non linear effect also comes into play.i would suggest you to jst change power and your system work fine .
Cheers !