If you use full cache, it has to download the whole. That's how it works so if you want to use full cache, you have to wait for the full download of the file.
There isn't anything to tune/fix on that as it just depends on how fast your Internet is.
You can use the cache backend if you want I suppose if that's what you are looking to do. What's your use case you are trying to solve?
VFS write mode shouldn't be any slower than full mode for small files.
Keep in mind that small files are inherently inefficient to transfer compared to large ones. It takes some time to request a file no matter it's size so even tiny files will not feel instant, and transferring a large folder of small files will inevitably not give efficient use of bandwidth. It should not perform any WORSE than full mode though...
I do agree that the PDF you demonstrate in your video is abnormally slow. 2-3 would be perfectly normal, but this is significantly longer. I can't point to any obvious cause, but I do not experience this personally (using VFS write mode).
full does a complete download of a file for read and write.
write does a download for "most" write activities against a file.
If you put the log into DEBUG and share the log, you can see what's going on.
If you have multiple opens and closes on a file, the cache backend is going to do well for you as you get the chunks local making multiple opens and closes quickly much faster.