So the keyboard plugs directly into the Mac with no hub. And then you have headphones plugged into some audio interface and there is a lag between when you play a not on the keyboard to when you hear in in the headphones?
What audio interface are you using. What is the size of the audio buffer.
Also the delay might be inside the virtual instrument. Have you tried different software instruments?
What is an 1/8th note delay? It would depend on the tempo and time signature. What is the delay in milliseconds? Or just tell us the tempo and signature.