[Tool] CompactGUI: Save Disk Space By Compressing Your Games

That was true of the older methods.

However, the newer algorithms used by Compact are much more efficient.

Testing is ongoing by the community but the general consensus (here, for example) seems to be that any performance degradation is negligible with this method with the titles so far tested (assuming your hardware is reasonably up to date). I personally can attest to this on the titles I tested myself. Exceptional cases are always possible, of course.

It always does, but there are compression algorithms that are super cheap to apply, such as LZ4 or LZO that are primary algorithms e.g. for compressing swap in zswap module on Linux.

In this case it's actually beneficial to use one rather than use no compression at all, since less data needs to be written on HDD (and read from it). The speed of compression/decompression is faster than the speed of reading raw data from I/O, so there is actually speed increase in this case. Compare this as if you'd read then decompress a zip archive written on 1.44 MB floppy drive, compared to reading 5 of such floppy drives for entire content. It's good to imagine this on real-use scenarios rather than in theory.

When it comes to compressing entire filesystems like here, then we mainly read, not write, so as long as algorithm is good enough then there will be little to no performance degradation, so yes, it has negligible difference for end-user.

(I'd still not use any solution like that since it's not in-filesystem compression but third-party tool that utilizies FS routines. I prefer fs-compression such as e.g. in ZFS filesystem on linux)

Not sure what the problem is but try the steps described in the Important Information of the Wiki.

Find some file in your system that is only some hundred bytes in size, and check it's properties.. It'll say something like Size: 308 bytes (308 bytes) and Size on disk: 4,00 KB (4 096 bytes). Now if you compress that file it's size may become smaller, but it'll still take that 4 KB from the disk. Now imagine that there are thousands upon thousands of files like these in your system, and compress doesn't really help at all with them.

Also, you really should defrag the disk after compressing the files. That'll help to make the freed up space available for use. Doesn't help with the issue above, though. (It's just how the filesystem works.)

Yeah, I noticed that about the Blizzard games, no real point in compressing those. The Ubisoft games don't compress as much as some others but I got a solid double-digit GBs after going through with it, all those AssCreeds and open world whatnot add up :)