Thanks. I copied whatsapp images folder to a different location and Mixplorer easily loaded 3600+ images from the new location.There are a variety of reasons why the view for a local folder might be populated slowly (and even more so for remote resources) including: number of files, presence of corrupt files or files in unknown formats, amount of storage space remaining, thumbnails enabled or disabled, etc. To eliminate possible causes you could make a local copy of the contents of the affected folder in a different location, if possible testing alternate locations on both internal SD card and external SD card.
Speaking of remote resources: even though it seems like the delay is reading that local folder, scanning and synchronizing between that local folder and a remote resource could account for the differences between what you and @Spannaa are seeing.
I think it has something to do with new android 11/12 storage feature.
I want to directly load whatsapp images folder faster.