Exporting (& rendering) extremely slow (Task Manager shows low CPU & practically no GPU utilization)

Because as RJL noted, the vRAM is maxed out. It can't "go" faster.

Unfortunately, 4GB of vRAM is really not enough for UHD sequences with effects applied these days.

Thanks for the reply.  Yes, I noticed that, but because other things/programs are simultaneoulsy working and runing fine, I didn't think that was exactly the issue.  I mean, I remember on my other systems I had the same amount of RAM and there wasn't this problem, but yeah, if that's really what it is, then I guess that's that.  I just didn't think that would  be the problem for rendering/exporting.

If the GPU VRAM is maxed out, how come there's barely (if any) load on it?

It looks like you do not have enough system RAM and graphics VRAM for your particular render job. The screenshot is so simple: Your main system RAM usage is almost maxed out while your graphics VRAM usage is pegged to the max. This will slam your entire system into the ultra-slo-mo operating mode for the remainder of your export job (very little CPU or GPU usage but nearly maxed out total RAM and VRAM usage).

In other words, you definitely need a PC with much more RAM and a GPU with much more VRAM for this particular job.