I am currently (forced into) using Microsoft's Photos app for viewing HEIC images on Windows, because it is faster than XnViewMP, IrfanView and Faststone Image Viewer for Apple's image format.
As far as I can identify there are three reasons for why it is faster:
- Photos uses progressive loading of HEIC images, which Xn does not for HEIC even when the option is enabled.
- Photos uses more aggressive (earlier) caching of next/prev images and also seems to keep more images in its cache compared to Xn.
- Photos may use less CPU load for HEIC than Xn to begin with. But Xn especially seems to hit a bottleneck that can even make it freeze completely without for multiple seconds when you flip through HEIC images quickly.
Images were loaded from M.2 SSD for this test.