Scrapebox slow down while running - why?

Hi!
When I start the scrapebox, it is very fast: avarage's URL/s is 1000/s
10 minutes later: avarage's URL/s is 500/s
30 minutes later: avarage's URL/s is 200/s
120 minutes later: avarage's URL/s is 100/s
Why?

Active threads: 400
Proxy: and 100.000+ free proxy
Keywords: I have more thousand simple keywords (apple, hotel, etc)
Harvester: I try harvest just the first top 10top-top100 results by keywords
PC: I have i7, with 8GB Ram, both are running 30-40 percent.

Thanks