4k Rotobrush Killing My PC

I've got a 30-second clip (799 frames) of a table rendered in DNxHR UHD HQ (3840x2160) that's roughly 2.8GB in size. The table is filmed with the camera on a monopod, so there is a little bit of gentle camera sway (desirable), but it's really pretty steady. I need to use the Rotobrush tool to punch the table out of the clip and place it onto an all-white background.

This is M-U-R-D-E-R-I-N-G my PC.

After I rotobrush the mask into the proper shape, rendering frames (propagating the mask on down the span) goes fairly quickly for the first 5 frames or so after which it slows down massively to maybe 5-7 seconds per frame. The more frames it renders, the longer the next frame will take. After it propagates the mask to the desired position on the span, you can forget about scrubbing across the green rendered area to see the results; it can take up to 10 seconds per frame to pull up for review frames that have supposedly already been rendered. That's as long as it took to render them (if not longer). Completing the rotoscope process like this takes hours on top of hours.

If that wasn't bad enough, attempting to freeze the brush strokes sends my PC into a rendering hell from which it usually does not recover. Trying to render this 30sec project takes multiple hours just to get up to 2/3 completion where it will inevitably give me an "out of memory" error and crash my display driver and/or my entire OS.

Now, it would seem to be very obviously the case that my PC is not beefy enough to handle working with 4k video. Here are my specs:

AE CC v15.1.1
Windows 10
8th Gen Intel i7 8700k 6 core/12 thread @ 4.7gHz
Nvidia 980ti
16GB DDR4 RAM
Samsung 960 EVO 500GB NVMe SSD

4xHDD 1TB RAID10 array

I understand that my RAM count is pretty low and 32GB is really recommended. However, my NVMe SSD (where the AE cache and my RAM pagefile is located) is literally capable of read/write speeds in the gigabytes/sec range and the RAID10 array (where the clip is located) can easily read several-hundred MB/s. I have tried putting the clip on the NVMe SSD instead of the HDD RAID and there is no impact on performance at all. Long story short, even though I'm almost certainly running out of RAM, my system should be able to read/write the ENTIRETY of the clip (let alone a single frame) nearly once per second without much issue.

I have tried watching my system performance in Task Manager as After Effects is rendering frames on the span, freezing the rotoscope brushstrokes, or attempting (and miserably failing) to scrub through rendered frames within a span. I was hoping to see a bottleneck somewhere, but I couldn't find anything. GPU utilization is in the low single digits. RAM utilization is, naturally, maxed out, but my NVMe SSD is hardly breaking a sweat with transfer rates of 40-250MB/s (most often around the 30MB/s mark). My processor utilization is a mere 40ish% and it isn't even bothering to turbo up to its max boost clock; it's just sitting at 4.3gHz. Occasionally (usually when scrubbing supposedly already-rendered frames), it seems like AE will attempt to call up an astonishing 800 MB per frame, which hardly makes sense.

In summary, it seems like AE is either hardly using my hardware yet still taking FOREVER to complete a task, OR it's calling on so much data per frame that it hardly makes any sense and it seems like working with this 30sec clip would require no less than 1/4 TB of RAM or more to even be properly functional. Could anyone help me understand what's going on here? Is my lack of RAM really causing all these issues, or is this just normal behavior for 4k video?

Thanks!