A lot changed under the hood with macOS with High Sierra 10.13. That alone could be enough for PS CS6 behaving slower than expected.
Are you running any 3rd party filters, plug-ins, extensions or add-ons? With any luck, it's just those as they're easy enough to remove or disable.
To further troubleshoot this, you could install the trial version of PS cc2018 to see if it performs better (it probably will). And/or, if you iMac supports macOS Sierra (it might not), you could create an alternate boot drive running PS CS6 under macOS Sierra 10.12. (A USB3 drive will work, but for speed I'd use a Thunderbolt drive instead.)