Same deal here. 10 bl brewhouse. It takes an hour to transfer through heat exchange using either hose size, I wondered the same thing, and tried it out. I think the hx restriction is the main factor, though the oxygen may be absorbed(?) into the wort better in the 1.5 inch hose - more surface volume ...
I set the vfd to about 25hz, and it goes roughly 1.5 bbl per ten minutes. Always bubbling within 18 hours - usually less.
Hope this helps,