It's not so much about the CPUs ( @igazor 's iMac is also quad core) but the graphics drivers as Parallels uses its own driver. Boot Camp uses Apple drivers initially but you can download the NVIDIA or AMD drivers if you wish which does make a difference. When Sims 4 came out I ran it in Parallels and Boot Camp to compare. Same save game, same graphics settings, same 2012 5" MacBook Pro, 2.6Ghz i7 (QC), 1Gb NVIDIA GeForce GT 650M, 16Gb RAM:
Parallels:
https://bluebellflora.files.wordpress.com/2014/09/parallels.png?w=625
Boot Camp:
https://bluebellflora.files.wordpress.com/2014/09/boot-camp.png?w=625
The Parallels image, whilst fine, looks flat and dull compared to the Boot Camp image.
