You can uncompress all the files on a Hard Drive from the command line on a drive that has all files compressed or that had selected files compresses. Open a command prompt and from the root of the drive (to decompress all files on the drive) or the root of the folder you want to decompress type the following.
compact /u/s/a/q/i
Compressed files take more time for the computer to process because they need to be decompressed and then recompressed as the file is needed or accesses by the system which adds to the load of the system slowing it down. Only compress files as a last resort to free up space. The best solution is to remove unneeded files or increase hard drive space by adding another drive or replacing the drive with a larger one.