New Mask Agonizingly Slow

I’m using new mask on an hour long clip that’s nested in a multitrack.  It’s inserting slides on a screen. Unfortunately, audience members heads encroach the screen area just a bit, so I have them masked in a sandwich timeline (base video | slides | masked video (copy of base)). It looks great, but when I go to the multitrack, it’s sluggish as all get out.

So I decided to render the sandwich timeline, and it says it’ll take 12 hours to render. It’s all CPU rendering, and the CPU is only being used 35%. 1-2% activity on the GPUs, and disk write speed of about 1.3KB/s.  I noted that tracking the mask over the one-hour timeline took almost 4 hours.

This is not acceptable.

 
System info
    Application: Premiere Pro (Beta) v26.2.0.18
    OS: Windows v10.0.26200, RAM: 63.79 GB GB, CPUs (logical): 16