Well could only go by title of post saying, downloads slow.
My 4 tests did seen to pause during processing but not more then a minute, usually even less tome, I did not check cpu use.
I Never figure why SF don’t also use gpu. Speeds up unifab and dvdfab.
You could try clearing cache, temp folder, history and cookies in SF.
Others may see something, because I not sure if I can test NF more, at this time.
Might be the video Outlander so that’s something others can try to check and test.
Also make sure temp, cache, etc are on a good speedy SSD brand name like WD.