This is not an uncommon complaint. and I don't really know the solution. I use .29, (for Mac) and I have a reasonably unique share which is about 3000 files. so is in pretty steady demand. I had reduced my upload limit to three files at a time, which seemed to help, but since I put it up to 4 and forgot to reduce it again, I have been having similar but not so drastic slowdowns. Also I tend to not leave it on for too long. A quick re-boot after several hours can be a good idea.
.30 is a fairly hungry beast and some people have been able to go back to .28 or .29 and find that things are much better. I think you must be using Windows, so dont ask me for help in that respect as I can't give it!
You still have 4.8.1 ......................