My RAW images are only about 15MB however I have zero delay or beach-balling even when editing a large number of files or using any of my Efex plug-ins. I have a 2011 2.8GHz i7 21.5" but upgraded the RAM to 12GB. I do NOT have a SSD and store my Aperture library on my internal 2GB HDD. RAM is a cheap upgrade (esp from non-Apple vendors) so it might be worth bumping up to 12GB (or even 16GB) considering the relatively low cost.
Did you use Migration Assistant when you set up your new iMac? I came from a 17" 1.83GHz Core Duo with 1GB of RAM and was constantly (and painfully) waiting when editing with Aperture. I did NOT use MA as I had read that it could also migrate problems from the previous machine. Not sure if there's truth to that but I set up as new and my new i7 machine flies (as it should). I did go back after setting up my new machine and erase the HDD from my Core Duo, reinstalled everything from scratch (set up as a new machine) and it performed significantly faster. Short story long - my old machine had some serious gremlins in it prior to wiping and starting new.
-andrew