Dan
I spent the morning running ground up tests starting with a single route with 70 orders.
I'll cut to the chase.
A VRP with 700 orders on 8 routes with a limit of 90 orders per route will complete in a couple minutes.
Basically I gave it a very small balanced problem. Orders are very tightly packed to minimize network thrashing.
But the same model where you adjust the order limit on the same routes to 875 will run forever. (I killed it off after 15 minutes - stuck on "Inserting orders 33%")
I got 1400 orders to run on 8 routes allowing 175 orders per route in 15 minutes. Pretty decent.
This shows that it isn't necessarily the orders, per se, that drive up time, but the function that growing orders requires you to grow route order limits which drives up time.
When the orders go up, which forces you to increase the order limit, regardless of number of routes, the time grows exponentially. When any one route gets too large things break down. The Network Analyst route order limit is 1000 orders per route. I was never able to get anything to complete with a limit of even 875.
Simply put 7000 orders on 8 routes is too big for the solver to handle.
One final note: I have it running a single route with 900 orders and a order limit of 900 and it has been running for over an hour. (the operative info. is that the order limit is high on the single route which makes it run poorly.)
If anyone can see a flaw in what I am doing PLEASE let me know.