This test is a real-world task using Apple XCode to diglloydTools.
The shocker is seeing a 4-core iMac 5K outperform a 3.3 GHz 8-core Mac Pro because Apple XCode uses all CPU cores.
There are two explanations for this astounding result; includes an analysis of CPU usage which explains how 4 CPU cores can beat 8 CPU cores.
One explanation (SSD speed) points out just how important the new Thunderbolt 3 standard will be to high performance systems: Thunderbolt 3 includes support for NVM Express, which will allow external SSDs capable of 2.4 GB/sec speeds.
More and more, the late 2015 iMac 5K looks to be Apple’s highest performing Mac for many purposes. The late 2013 Mac Pro, even a 3.3 GHz 8-core CPU, is looking positively geriatric in performance terms.