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I would love to publish the actual numbers and details someday, but for now, here's a handy tip I learned the fun way from Guild Wars 2: if you use precompiled headers for C or C++, structure them carefully and monitor how much disk IO is spent reading and writing them in an average build. You may be horrified.
Corollary: not 100% of the storage used by programmers and build machines should be solid state. Testing on platter drives will be important for "minimum spec" compatibility for at least 5 more years. If you always build/test from SSDs, you may become severely IO bound on older drives, and performance differences can be over 100x.