Image sequence with no effects not rendering multiple frames simultaneously

I have a bunch of PNG image sequences that I'm trying to preview in AE and it won't render multiple frames simultaneously.  There are no effects applied, only a scale.  Successive frames have no dependence on each other so I don't understand why it can't render several at the same time.

My CPU is i9 14900k with 128 GB RAM.  I have AE set to render multi frames simultaneously but every time I render a preview it chugs through the images one-by-one, using very little CPU and taking forever.

How do I make AE render multiple frames simultaneously (like it did back in the day...)?