Long engine processing time when orders mixed with priority

[gibhsu]

We have orders with priority. When we executed FrePPLE with mixed priority, it was an extremely long time.
However, when we only loaded the orders with the priority 1 and priority 2 & below in separated Runs, FrePPLe was super fast.
Just wondering what could be the root cause?

Here is the order quantity
priority 1: 804 orders, priority 2 and below: 379