Hello,
there is hidden feature in LibXISF that is used by INDI and KStars. That you can override compression when saving XISF files.

You can set LIBXISF_COMPRESSION environment variable to these values.

• zlib
• lz4
• lz4hc
• zstd
• zlib+sh
• lz4+sh
• lz4hc+sh
• zstd+sh

This set which compression method should be used. "+sh" variant are doing byte shuffling which further increase compression ratio. Best place is just open "~/.profile" file and add line new lineOr you can just enable compression in INDI control panel in your camera settings. It will use either zstd+sh if available or lz4+sh.

Next version of KStars will support ZSTD which have really nice compression ration where my Lights are 10MB instead of 32MB. ZSTD compression is also really fast that even on RPi it add only slight delay.
To utilize ZSTD you will need latest version of PixInsight. Also support for XISF is coming into Siril so next version after currently released 1.2.0 will have support for reading XISF files.

Sounds good. When you say your lights are 10MB instead of 32MB, is that zstd+sh vs fits?

Yes zstd+sh compressed xisf is 10MB vs uncompressed fits/xisf 32MB.
zlib+sh have good compression but it is slow. Even on my desktop Ryzen it takes several seconds to compress one image.
lz4+sh is fast like fraction of second to compress one image but compression ratio is not so good.
zstd+sh is best of both. Slight slower than LZ4 while almost as good compression ratio as zlib.