There’s not much to do when it comes to configuring your iMac for audio recording. One potential problem is the “Fusion Drive” that has been causing a lot of people headaches.
What is the fusion drive? Fusion is one of three storage options: ATA, Fusion, or Flash
“Fusion Drive combines 128GB of super-fast flash storage with a traditional hard drive. And now it’s up to 50 percent faster than before. It automatically and dynamically moves frequently used files to flash for quicker access. With Fusion Drive in your iMac, booting, copying files and importing photos are faster. Over time, as the system learns how you work, Fusion Drive makes your Mac experience even better. All while letting you store your digital life on a traditional, roomy hard drive.” (Apple).
You want to either stick to the traditional ATA hard drive or, better, go with the flash storage option (as much as you can afford). If you purchase 256 or 512 GB of flash then buy a USB 3 external hard drive to store all your project files.
My new iMac has Flash memory and I use a WD USB3 external hard drive and the performance is killer and problem-free.