02 Oct 2016
Well... I now think I gave the impression, REs need a lot of CPU in general. Thats not true. There are a lot of REs that will run on your 2013 iMac. I have used REs for a long time now, and mostly on my MacBook Pro. Its just a few REs that require so much CPU. EXpanse is a monster CPU hog, The FM4 is not as bad, but still requires a lot of CPU. There's a bunch of them.
Best is to trial them before you buy to see how them work with your system, a LOT of REs will run on your config without troubles. Remember, I am on a 2-Core i7 mobile processor from 2012, you have a quad-Core i5 desktop CPU from 2013, I guess your mentioned iMac is about twice as fast as my MacBook Pro. You don't need the latest and greatest Equipment to make good House Music. A lot of stuff you can do with stock devices. Stock devices are great in terms of CPU usage, You can easily run like 20 subtractors on your mentioned config and then some and then some again.
Antidote will run with a few instances as well and thats a killer synth, KHS One will work easily, Red Rocks Ivocs and RED70 will work and the list goes on and on. Same is true for effects, most of them will work fine even with multiple instances running.
The trick is, to use those REs everywhere you NEED it because of the specific sound or their features. For everything else use stock devices. With that strategy you can run 20-40 devices with no trouble