I use AME on several different PCs and one produces extremely long render times. That one also shows very low CPU usage when encoding via AME.
For example, a project on one machine (i9 7900x) has a render time of about 40 minutes and CPU usage of 75% - 85%. The exact same project on an i7 6800k uses only about 11% CPU and the render time is about 3 hr.
Of course, the two machines have different numbers of cores and different graphics cards, so that will affect the render time. But why does the one machine use only 11% CPU while the other uses over 75%? Certainly that fact also has a major impact on the render time.
The fact that the CPU usages are so different seems to indicate that something is not configured properly in the machine that is using only 11% CPU. Both are using the latest version of AME and both have all the same options selected in AME.
It also depends on many other factors like permissions, location of footage, export location.
Please check the following articles for optimization; the articles are applicable for all Adobe video applications: