Lightning took out eathernet port. will it be faster down loads with wifi or eather net to usb adapter

Depends on your model of iMac which determines what WiFi

it is equipped with and what type of USB it has. It also depends

on the type of internet connection you have.

In all likely hood, it is possible the internet connection will be

the determining factor.

If your system has Thunderbolt, then I would suggest Thunderbolt instead of USB. I've used both on older systems and Thunderbolt is faster, but the answer depends on the exact model of your system. Whether either of these options is better than WiFi depends on what WiFi standard your current router supports.