That may be your experience but I am not sure it's a general issues and, as stated, there is no reason why SD should be the cause of delays.
Lat week, my SD item arrived after 2 days, this week, a 48 hour tracked item took 3 days. That is not a 'massive delay' though.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)