They've been working on the Steam client recently and one of the fixes related to the fragmentation of game files. It's always been a feature but it was never really used - after the update is wanted me to defrag TF2 but I think that was the only one.
There's a new Steam client beta (accessible under Settings) that's probably worth trying - I've been in the past half dozen or so and never come across any issues, plus you get to check out all the changes before everyone else.
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Founder of the "I Hate Smiley Fitz"
Riley has autism. He has trouble communicating, and in an overstimulating
environment, he can get frightened and run away, leaving his parents frantic. - Auburn
"The price of freedom is eternal vigilance."