It may be that Opus, on a separate thread, is trying to populate the lister with information by inspecting the files. This will happen if you have the Description column turned on, or any other such as image width, music artist, and so on, which requires looking inside the file. Of course, thumbnails mode will do the same thing.
If that's taking a long time to happen then when you try to delete files Opus may be tripping over itself.
You can removed the Description column from network drives (see the Folder Formats FAQ for how to do this). I've done this myself, athough I did it to avoid wasting time and network resources populating the column, rather than to avoid this particular problem (which I've only seen once or twice).