What is 'System and compressed memory' and why is it using so much memory? [duplicate]

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.