You are limited to what Apple used in your system, so looking at EveryMac listing for your model: [https://everymac.com/ultimate-mac-lookup/?search_keywords=iMac15,1|iMac15,1]
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You are limited to what Apple used in your system series, so looking at EveryMac listing for your models listing: [https://everymac.com/ultimate-mac-lookup/?search_keywords=iMac15,1|iMac15,1]
We can see the 4.0 GHz Core i7 (I7-4790K) is the best CPU Apple offered.
You are limited to what Apple used in your system, so looking at EveryMac listing for your model: [https://everymac.com/ultimate-mac-lookup/?search_keywords=iMac15,1|iMac15,1]
We can see the 4.0 GHz Core i7 (I7-4790K) is the best CPU Apple offered.