I copied lots of files from a friend's external drive and my computer has slowed down, why?

A few weeks ago a friend gave me a considerable amount of files through his external drive. Afterwards I created a quite a few folders to organize the files and deleted those I didn’t want to use. From the 13 Gigs that I’ve copied I finally kept about 7. Ever since I did this though my computer has slowed down... show more A few weeks ago a friend gave me a considerable amount of files through his external drive. Afterwards I created a quite a few folders to organize the files and deleted those I didn’t want to use. From the 13 Gigs that I’ve copied I finally kept about 7. Ever since I did this though my computer has slowed down noticeably. Can you give me a hint? I don’t understand why everything became so slow!
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