I've also noticed a similar issue with Affinity Photo and when working with raster layers in Designer (1.8.5, but I noticed it on previous versions too).
If I select a group of layers and try to move them around, I'll easily exceed 75% CPU usage and even spike to 100%; performing the same task with the same file in Photoshop does not exceed 10%. In terms of actual performance and snappiness I don't notice a huge difference between the two programs, but the burst of fan noise and heat is definitely jarring.
I note that this issue is particularly noticeable when manipulating layers (moving them around, setting opacity and visibility, etc.) and zooming; I'm not as concerned with CPU usage relative to Photoshop when doing tasks like running filters, blur, opening files, etc., but the spike that comes with common operations like layers and zoom seems excessive.
For tech info, I'm running Windows 10 2004, an AMD Ryzen 2600X on an Asus B450M, default CPU clock and fan settings except the RAM (32GB @ 2933mhz), and a Radeon RX580 (GPU rendering enabled); the file I tested was a 6600x10200px PSD, RGB/8, 5 raster layers.
Hoping this is something that can be investigated, thanks!