Files being uncompressed faster than physicaly allowed by hard drive?
Solution 1:
How much RAM have you got in your new computer and what size files and OS? The speed is most likely down to it all happening in RAM before it hits the disk depending on the size of the files and what the app is you are using to decompress.
Solution 2:
Csv should compress very nicely, so if it compresses 4x, then it can be read at 50MBps but decompressed at 200MBps (as it expands 4x), and if it isn't too large, can then be buffered before being written out to disk.