Well the Mac air given that it is WIfi has a max speed of what you have indicated unless you bought an adapter that allows you to attach an ethernet, that is the max speed of Wifi AC if you have an AC router.
On your iMac, I’m guessing that you have wifi turned on too… Go turn it off and make sure that only ethernet is turned on.
Let me know if that was true… I’ll wait here till you check… (I waited awhile and had to go back to sleep… see you in the morning). But just a side note on this, I once had both the ethernet and wifi settings on and my speed defaulted to wifi, so that is how I found out that I got wifi speeds instead of my gigabit ethernet speeds.
Okay are you sure your iMac has a gigabit ethernet?
go Open Network preferences, select your ethernet from the left scroll, then click on advanced, click on tab hardware and see what the speed is?
