Q: Windows and Apple?

Faster in what way? The only way the MBP could be faster, is when the data on the desktop are coming from the second drive (unless that's a SSD too). Or when there is some kind of hardware failure or software limitation.

Same version of Photoshop? And do both have the same settings? Nowadays a lot of rendering is done by the GPU. If that option is switched off on the desktop pc, it will be much slower.

(Anders zou ik het ook niet weten! )

Maybe your observations are influenced by the fact the computers are not running similar generation OS. Windows 8.1 has been replaced by the current Windows 10. Your speed observations are anecdotal. What tools did you use to accurately capture the differences? Windows and Macs run at similar speeds if all specifications and apps are the same.

Not sure why you're asking that here. As the issue lies with the Windows machine I'd be asking on a forum for that OS or Hardware.

Wild guessing ...

at first you should check your desktop for viruses, malware, adware. Have a look at Baidu Antivir.

At second run a file system integrity check. (My pc, right click to the local disc symbol, properties, Tools/check AT FIRST/ then optimize, or properties / common / clean.

DONT TRY ANY KIND OF CLEAN MY PC crapware.

greetings from germany