A lot of that iMac's cost is its retina display, and you didn't bother buying a retina display for the PC. Running a speed test is silly, there are already major differences in the build. To make this remotely more scientific, you'd either need to buy a 4k monitor for the PC, or use a Mac Pro and just take the monitor out of the equation.
But the Mac pro hasn't been updated in years. The gpu is old, and this is mostly a gpu test.
The article states repeatedly that it's all about CPU clock speed. If that's inaccurate, could you elaborate on how it's actually a GPU test?