My device is Samsung Galaxy A50 running Android 11. This sluggish loading bug occurs with files from Internal Storage and SD card.
The MiXplorer version without the bug I'm using now is 6.57-API29 B21121050. The version with the bug is 6.58.4_B22020910. With or without MiX Image installed, same result.
I attached 2 clips: a clip with how slow MiXplorer loads the images on 6.58.4_B22020910 and another clip with how fast MiXplorer loads the images on 6.57-API29 B21121050. Same images, same location. No MiX Image codec installed in both cases (even if it's installed, same result). I cleared the app's cache before both tests.
If you want to dig further you could try testing with the files in a different location on the internal storage or on external storage (slot in device, not usb-otg).