My first guess... and it could easily be wrong...

I assume that you are talking about 650 - 700MB movie files...
After they are downloaded completely they are 'hashed', right ?
That takes a bit of system resources !
And, perhaps you are also running a virus-scanning application that should also be checking the new file at the same time.

Big Time CPU usage...
If you are doing anything else while these processes are running, slooooooooow will be the mode !

As I said, the above may not be the problem, but it is sure worth checking it out...

__________________
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