Why is my PostScript print quality so poor?

Solved it. The pages were being rendered on the computer because I had chosen a LaserJet driver from the Hewlett-Packard section of the CUPS printer management interface, implemented using Gutenprint. This worked fine for B&W text but poorly for color areas.

After still not seeing a "PostScript" option in the configurator, I chose "Raw", and this works: now the rendering happens in the printer, making the paper output look proper with both graphics and text. Thanks to all for your help!

Seems you might need to install HPLIP proprietary driver plugin:

https://developers.hp.com/hp-linux-imaging-and-printing/models/laserjet/hp_laserjet_400_mfp_m425dn