Upload to iCloud takes ages

Saved 4 files to the folder on the desktop. First three files uploaded in a second (300mbps upload link). Fourth file was uploading for several minutes. Even after I added two additional files - they weren't uploaded, but they were waiting for the fourth one to be uploaded, so there was a queue, where this one file blocked whole sync for minutes. This was one photo copied from Apple Photos, HEIC file, named by iPhone which took this picture, not some kind of strange file.

Why is that happening? It happens a lot, several times a week. What is wrong with this service> How to resolve that? I spent like 5 hours with Apple Support and they don't know. Is there anyone in the world that knows? That is really, really annoying for a paid service (I paying for additional storage).

Here's the screenshot, don't worry about it saying 2 items, because every time I click the (i) icon it says something different, sometimes 2, sometimes 4, sometimes 3, whatever...