Hey, i've had so much struggle and problems working with After effects and sometimes with Premiere as well. I'm using 2016 Macbook Pro with 2,7GHz i7,460 Radeon 4GB,500SSD and 16GB RAM. Recently i had to work with audio in Ae and it just lagged so badly that i couldn't work. One of my friends helped me out with this and it's kinda stupid but that's how it is anyway. If you are experiencing something similar or need that extra power then follow these tips:
Only on Macbook's * because iMac's are using power sources.
System preferences->Displays->then you should have options to scale the resolution(check picture below) and apparently Macbook Retina display needs so much resources that it can't handle everything at once on maximum resolution,so i tried to lower my resolution as you can see on the picture and after that my Ae was at least 30%-50% faster.Let me know if this helps you as well.