There are many issues with M.E. 2019, this is specifically in regards to it having severe effects on responsiveness of all other applications when encoding. In my case, it's an AE project. I'm on a 2017 iMac Retina, on Mojave 10.14.4 with 64gb ram and a Radeon Pro 580. I have tried both Open CL and Mercury Metal encoding. No change.
I get beachball or just a stuck cursor for 2-3 seconds every 5-6 seconds, making the computer pretty much unusable while encoding. Typing this post took a half hour. Many of my projects use Element 3D, but I have always been able to use M.E. with very little tax on system prior to 2019.
I have uninstalled, reinstalled and had multiple updates on both AE and M.E. and wiped preferences, no change. (though it did fix some other problems)
In system activity I notice the GPU is almost completely maxed out while the CPU usage and memory usage is almost none.
I'm open to work-arounds, but downgrading my AE isn't an option as most of my projects were created in 2019.
Have you updated your GPU drivers? If your system is becoming unresponsive during render, I would think it's maxed out Ram, but if you say utilization is low during render then that's strange. Maybe try playing with the Preferences > Memory settings as well to see if that fixes anything.