Jason wrote:
In article , lid says...
1) Compression is slow.

Yes it is, but tweaked lzw compression allows for very fast expansion.
Years ago, IBM added some instructions to the RISC/6000 to perform it.
RISC instructions have less entropy than CISC instructions, so it's a
win.

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Apparently the Compact command has been given four
algorithms, but it doesn't say here which one is
used for the /CompactOS option.

https://winaero.com/blog/ntfs-lzx-co...hm-windows-10/

LZX
XPRESS4K (fastest) (default)
XPRESS8K
XPRESS16K

(LZNT1 is the old compression method as a built-in
LZNT1 uses no Reparse Point per file.)

In Win10, if you issue

compact /compactOS:never

and if the files aren't compressed, it takes forever and
the command returns

0 of 17983 files decompressed

In other words, it takes a while just to check for
Reparse points on all the files. I think it's possibly
the overhead on selecting the files, that makes it slow.

And I've had Cleanmgr.exe spend two or three hours
compressing those files. Whatever method they were using,
there must have been a complicating factor (Windows Defender
scanning each file too?).

The compressor that handles WIMs seems to be in the
same class as 7Z. It takes a while, and the compression
achieved is pretty good. One way to test that, would be
to see if 7Z extreme setting, can make storage of Win10
ISOs smaller.

Paul