OS X suddenly becomes very slow
It turns out to be a hardware problem. After getting frustrated with the extremely slow response, I reinstalled OS X via the recovery tool. But that doesn't last long - it starts to get slower (but not as slow as this case), and the Disk Utility complains that "This drive has a hardware problem that can't be repaired." So I send my MacBook back to Apple and they replace the hard-drive for me and it was covered by warranty.