from my understanding it has to do with the types of cards sent.
CGC has modern graders, and they have vintage graders. (yes, some can grade both but i'm assuming there are more modern graders than vintage)
there are also graders that are more specialized in different TCGs, sports, etc.
Some graders are more backed up than others. High value cards probably get looked at by more graders.
a lot of variations here.
there is a point in tracking turnaround times, but maybe we should specify what exactly is in our subs.
my 3 card economy has been in GEI a little over a week now. i see some subs fly in and out of GEI. i'm assuming mine is taking a bit longer because i have a 2004 animal crossing card, and there are 0 pop. other 2 cards are vintage japanese pokemon and vintage yugioh.