The late 2012 iMac stomps the fastest Mac Pro system for common Photoshop tasks, so long as it has adequate memory.
But for really big jobs the iMac lags the Mac Pro badly because the maximum memory the iMac can accept is 32GB. As well, there are other considerations for general use besides RAW CPU speed.
See the multiple pages of my review of the late 2012 iMac 3.4 Ghz. More to come.
Click each graph to read more on that test.
An iMac with only a hard drive would do much worse than shown here. As shown here for Fusion numbers, an empty Fusion drive performs very well, but once more than 120GB of capacity is used, its performance plummets, because the Fusion technology maintains only 4GB of free SSD working space.