File transfer from iMac 2009 running El Capitan to new iMac

Connecting you old iMac's Time Machine backup and Migrating during the initial setup, is the fastest and easiest way.

If you do not have a Time Machine backup from your old iMac, then using an Ethernet connection is the next best way to Migrate your files.

see > How to move your content to a new Mac - Apple Support

Very simple to do, just connect the old Macs Time Machine External Hard Disk using a USB cable before starting the new Mac. Turn on the new Mac and it will startup with Setup Assistant and eventually get to a point where it asks if you want to migrate from another Mac. Select Yes and follow the on-screen prompts and let it migrate, typically it is done in less than 2 hours.

Wifi would be very slow& iffy.

Do NOT attempt a WiFi migration, they fail almost 100% of the time, use a simple USB cable to connect your Time Machine drive to the new Mac.