Why isn't Media Encoder trying very hard?

Adobe Media Encoder doesn’t seem to be trying very hard. I don’t understand why. Here is my setup.

Hardware: New Desktop Mac Pro (2019), 12 cores, 384 GB RAM, 4 TB internal SSD, AMD Radeon Pro Vega II Duo

OS: Catalina 10.15.5

Software: Premiere Pro 14.3, Adobe Media Encoder 14.3. Adobe is allowed to use 330 GB RAM

Everything is up to date.

I created a PP project with a 2-hour timeline, no transitions or effects. Output queued with these settings:

Vimeo 1080p preset, hardware encoding, Profile: High, Level 4.2, 1 pass VBR, 16 Mbps

During AME encoding I used Activity Monitor to check out what was running.

CPU usage 18%

GPU usage 13%

RAM used 30 GB

Disk: 15 MB/sec written

It takes 18 minutes to encode a 2-hour timeline but it is hardly using the CPU and GPU and it has tons of unused RAM if it wants more. Why isn’t it using all the computing power available? No other apps are running on the computer. I also tried saving the PP project, closing PP, and then opening the project in AME so PP wasn’t even open. Same result. I have measured the SSD speed with Black Magic and it is 2.8 GB/sec read and write. I don’t see any bottleneck. It is not using any resource anywhere near it’s limit. Why is it so slow? Thanks.