I copied some 2900 MP3s from my player's micro-SDHC (or SDXC or what they're called now) to my Win8 convertible last evening.
Did the same thing on my Win7 computer two weeks ago because my car radio will not take micro-SDHC and I'm tired of fiddling that tiny thingie into the adapter every time. So I copied the whole lot to a normal-size chip.
The source chip isn't particularly fast (not needed for playing back MP3s anyway), specced at 25MB/s read (manufacturer's dreams, as usual).
Win7 starts copying SDHC-to-local-disk at 20MB/s and slowly degenerates to 16MB/s average over time. Whole process took 18 minutes.
Win8 starts copying SDHC-to-local-disk at 20MB/s and keeps this up for about 15 seconds. It then drops into a read-16MB-write-sync-sleep mode. The very detailled copy dialog alternates between 1-second periods of "20MB/s" followed by 1-second periods of "0 bytes" with a second or two of transfer rates in the hundreds of kilobytes in between. Whole process took well over 2 hours.
The Win7 machine is admittedly a desktop computer, but according to the "Windows performance rating", the convertible's SSD can very well keep up, and the convertible was running on AC power all the time (so, no, it didn't go to power save mode in between!).
After putting in the other SDHC (which normally serves as the secondary disk) again, Win8 told me there was "a problem" with the disk, and it wanted to "repair" it. The disk had been removed properly, and there was no fucking problem with it at all. The chips is as good as new, too. It had about 22% of its capacity written once.
Good job Win8 doesn't tell you what "problem" or "repair" means either (probably means running checkdisk or another useless utility for 2 hours). So instead of "repairing" it, I put it into the WIn7 machine. Win7 accepted the chip without complaining, all data was accessible. Inserting it into Win8 again gave the same error message, but it works flawlessly.