[justlurking]

I would rather Apple apply bzip compression to the files I haven't used, before uploading them to iCloud. So, it would be a three-tiered system:

1. Files I've used in the past month: Uncompressed
2. Files I haven't used in the past month, but my disk is full: Compressed. This can be done by a background task.
3. Files I haven't used in the past month, but my disk is full: upload compressed versions to iCloud.

On average, file compression reduces disk space by about 50%. But, it depends on your set of files: text files compress by about 90%, but JPGs don't compress at all.

Microsoft offers the compression option in the disk cleanup utility.

On an actual disk drive, you'll never notice the difference: the CPU can decompress the file faster than it can be read from the disk. On an SSD drive, you might notice a delay the first time a file is opened in 30 days, but after that it will be uncompressed.

[Swordmaker]

On average, file compression reduces disk space by about 50%. But, it depends on your set of files: text files compress by about 90%, but JPGs don't compress at all.

JPGs are already a compressed format. That's why you cannot compress them any more.

Modern Macs use Solid State Drives and they are VERY FAST. You can access the far faster then the CPU could decompress the data from a Hard Disk Drive. Otherwise, good ideas.