Why 7z compression speed and CPU usage are low
Solution 1:
By checking without compression you already confirmed the reason: you're not limited by CPU power but rather by IOPS. If you truly do have a hard disk (spinning rust), this is speed (40+ MB/s reading + 40 MB/s writing + seeking) is acceptable. In your screenshot, the destination is not on your SSD. Your hard drive may also be busy otherwise with regular operations.
It appears Windows Server does not show the hard drive in Task Manager but maybe you can see it in Resource Monitor and confirm that it is in fact ~100% busy.