"I've started the download but there aren't many people who are offering to share the file which means the downloading process will be slow.
I doubt my connection can upload all 717Mb of the movie so I will have to give you the video via USB. All will be done on the assumption that I can download this video on time."
I have a friend who wanted to know if these sentences were grammatically correct. I underlined the part I wasn't too sure of.
Red = Diction
Blue = Subject-verb agreement
The first sentence needs a comma after 'file'. Otherwise you are are talking about "a file which means the downloading process will be slow". Such a file probably doesn't exist, and if it did, it is probably not what you're after.
That is, you need a comma to convert this defining clause into a non-defining clause.