Extremely slow HDD (read and write)

Hello. I'm having problems with an almost full (97% full) Western Digital 1.5 TB 7200 RPM HDD. If I try to read from it or write to it, it takes so long it's practically unusable. Even just loading up a directory in Windows Explorer may take multiple seconds. I ran HD Tune Pro and the Health tab said the drive has unstable sectors (Number of unstable sectors: 1456), so I ran chkdsk /f, but...