Better to change rim or tyre size to get lower rolling resistance on trekking tyre? Changed back tyre 700x35c (WTB ALLTERRAINasaurus) to 700x23c (Schwalbe Lugano) and front rim from 18 to 13 inner width (photo below), is it better to change rim or tyre size to get lower rolling resistance ?

One of changes made significant improvement (>20% shorter travel time), but not sure which one.



Pressure around max 500kPa front, 700kPa back (max. >8bars)
The tire change would have been the biggest change, the ALLTERRAINasaurus is a knobby tire, while the Lugano is a slick road tire. 

Years back I used an ALLTERRAINasaurus and it had to be one of the slowest touring tires I have used in my life. Some earlier versions I later found out also had an incredibly stiff casing making it even less efficient.  

I have also ridden on Lugano as well. It is a slower tire too, kind of like a gatorskin, but no where near as slow as the ALLTERRAINasaurus.

Note that  your pressure could be an additional confounded factor, that said from my experience with both tires I would put money on the the tire change being the culprit. The rear wheel also carries the most weight so will be disproportionately affected by rolling resistance.