windows 10 - What is 'System and compressed memory' and why is it using so much memory?

On Windows 10, There is a process System and compressed memory.

What is it and why is it using so much memory?

Answer

The answer is in the name,

System and compressed memory

A quick explanation of this can be found here

In Windows 10, we have added a new concept in the Memory Manager
called a compression store, which is an in-memory collection of
compressed pages. This means that when Memory Manager feels memory
pressure, it will compress unused pages instead of writing them to
disk. This reduces the amount of memory used per process, allowing
Windows 10 to maintain more applications in physical memory at a time.
This also helps provide better responsiveness across Windows 10. The
compression store lives in the System process’s working set. Since the
system process holds the store in memory, its working set grows larger
exactly when memory is being made available for other processes. This
is visible in Task Manager and the reason the System process appears
to be consuming more memory than previous releases.

Remember that unused RAM is of no benefit to you. Not having to write to a page file and read it back again from a significantly slower storage device is a faster method of memory management. Note that if you do need this RAM, the system will release it or continue to use a page file as required.

In short, this is expected behaviour of Windows 10, not a bug, and will provide speed benefits over simply throwing everything into a page file.