Does it import more than one ppd files too? I've tried to import at least 4 ppds for different printers in the lasts 20 years. Most of them with the IBM cli tools before your versions come up. I'm thinking about if it's worth starting from scratch again.
Sure, you can import more than one PPD. Every PPD you import is saved in \sys\install\prndrv\ppd_
xx and kept whenever a new import is done.
Btw. I've sometimes still the problem with your psndrv (?) that it takes very long until the page came out. Sometimes pages are printed only after a reboot. Also I can't still print with small margins (less than 12,xmm). Although one of my printers allow 4-5mm the other can print without any margin on the other system. Maybe my setup is mess up hence the question to the expert on this topic :-).
Margin capabilities are defined by the PPD. Laser printers as a rule have a minimum margin size. Only certain inkjets can do margin-less printing TTBOMK.
Performance could be caused by any number of things. The first thing I'd check is whether the IBM PSCRIPT driver behaves the same.
One thing to try is reducing the print job size by tweaking the font download settings (this is highly dependent on what type of job you are printing so there's no set rule for it; try printing to file and comparing the file sizes with different settings).
If the printer's Postscript emulation is simply slow, you could also try printing as image (some apps like Lucide have a setting for this).