Is your image in a pyramidal format? If not, that is probably the cause of the problem. Whole slide images need to be in a pyramidal format to be used efficiently in Qupath (or other major digital pathology / image analysis software for WSIs). Major slide scanners usually generate pyramidal images by default. I’m not familiar with the Keyence BZ-800, but you may need to save the images in a different format or convert the images somewhere else.
“although OpenSlide and Bio-Formats support many TIFF files, it is quite possible to encounter variants that cannot be opened with QuPath. Perhaps the most common reason for this is that the file does not contain pyramidal layers, or these layers cannot be automatically recognized. This is one reason why well-supported, open formats should generally be preferred (e.g. OME-TIFF).” https://qupath.readthedocs.io/en/latest/docs/intro/formats.html