Right click on the bar at the top and chooseCompr.and it will show you the compression ratio.
Windows will compress some things irrespective if you turn on OS compression or not. It is also nothing whatsoever to do with NTFS compression.
My ratio in the picture above is slightly higher than default as I deliberately compressed .NET and the fonts directory which Windows doesn't do.
It's not just free space that determines if the OS is compact or not. Operational speed comes into the calculations.
I have two near-identical Dell Latitude i7 laptops, both had a clean install of 1803, subsequently upgraded to 1809. The upgrade compacted the OS on one, but not the other. The only significant difference in specs is that one has an SSD and the other has an HDD.
1809 seems to have determined that the time saved reading less data from the 'spinner' more than makes up for the time taken to decompress on the fly. I'm leaving it that way, seems to work well.
SSD system:
The system is not in the Compact state because Windows has determined that, it is not beneficial for this system.
HDD system:
The system is in the Compact state. It will remain in this state unless an administrator changes it.
Similar here. i7-7700K SSD compacted, virtually identical OS on i5-4300u SSD did not. Whopping difference in CPUs, not SSDs. So you are correct, the performance trade-offs are taken into account.
Which is fair enough. In fact you could argue (and my personal anecdotal test supports this) that compression is always better.
If you think about it it would only be worse if your CPU was pitiful or otherwise occupied. If your CPU isn't already generally at 100% utilization then from a performance point of view (although not power consumption) you better use compression.
I found out that if I update W10-1803 (with the compression issue in this post) to 1809 using MCT, the result is without OS compression. Beats me why, but it works.
However, if I update using "Check for updates", it fails and OS compression remains.
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