Migration assistant takes forever via WiFi. Is there a better way (cable connect old and new?

Wi-Fi migration will actually fail much of the time and an Ethernet connection is only slightly better. The best and fastest method, is to connect your old iMac's Time Machine backup to the new Mac and Migrate from it.

Then as tbirdvet suggested get an external drive and create a backup according to this Apple Document: Back up your Mac with Time Machine - Apple Support

Then make one.