- Newer more powerful iMac.
- Move off Aperture to Lightroom, Capture One Pro, or other DAM with non-destructive editing. Apple abandoned Aperture and you should also.
I agree with both of these steps, though your iMac is pretty decent with adequate RAM and an SSD. I assume that you are browsing images that are on the SSD and not images on a slower external drive or, worse yet, the camera's memory card.
Upgrading from the antique, abandoned Aperture to a viable DAM that is continuing to be developed and supported is a good idea, period.I'd recommend doing this a step #1.
Upgrading the older iMac will help somewhat, but disk speed and RAM are the most important attributes, but I'd first test the newer DAM on your old iMac to see if an upgrade is really needed at this time. A newer processor and motherboard components (memory buss, video controller, ...) will help only moderately. I you are using images stored on an external drive rather than the internal SSD then a new iMac can offer significant improvement if, and only if, you also upgrade your external drive to either a USB3 or, better yet, a Thunderbolt interfaced SSD.