Just stumbled onto a recent analysis of the current generation, top end iMac (2016) and an equivalent cost (~$4,000) custom built PC, pitched head-to-head in a selection of Lightroom tasks such as photo merges, importing files and on-the-fly thumbnail creation. You can read the full break down here, but the general gist probably shouldn't come as any surprise: the PC won, categorically, across all tests.
To be clear, this was a relatively unscientific, simplistic set of tests that purely focused on the minimum time required to complete a task, but given that time wasted is arguably the greatest commonly felt issue with post processing, I also think it represents a valid and fair test. I would have liked to see basic system usage stats also recorded alongside, however, as it's one thing to argue that the PC is faster, but if it's also unusable whilst performing the task, personally, that would be more irritating. It also doesn't look at multi-threading (another big time saver) or any similar multi-tasking, which I would be intrigued to analyse. I'd still predict that the PC would outperform the iMac on both counts, but I imagine the difference would be less drastic.
I'd also like to see a similar comparison run on more budget friendly models (although I imagine this would fare even worse for the Mac) and on similar laptops, the one category I would assume the Mac(book) to take the prize in. All that said, I'll still argue that Mac's are often a more useable tool for a photographer, simply due to the wealth of software available and build quality, however personally this article just highlights that they remain entirely too expensive an investment for the return.