As far as I know OS 9 can't utilize the second core and despite (I think) Photoshop there were no OS 9 applications that did, so a Single 1,8GHz CPU Upgrade or other would do as well (they can be found much more often and often go for 50,-EUR here). Actually a Sonnet 1,8GHz CPU (type 7447) is on par speedwise with a 1,25-1,42GHz Apple CPU (7455), that can be found in the PowerMac G4 MDD
https://everymac.com/systems/apple/powermac_g4/index-powermac-g4.html - His iMac G4 has a 1,25GHz 7445 Apple CPU, which should be slower, but I don't think that is the problem here anyway:
If a Mac is capable of booting into OS 9 every CPU speed will do OK (of course there are scenarios, where a faster CPU will be nice, but...)
If a Mac is not able to boot into OS 9 and has to emulate it via the classic environment you need a fast CPU, but then again you won't be able to do heavy stuff anyway in classic mode. (I never tried Photoshop on a G4 in classic mode, but I assume it will be a pain no matter the CPU speed. But then again Macs can always act surprising

).
I had some strange experiences with Classic.
- iMac G3 DV 400MHz, 1GB RAM, 10GB HDD with 10.2.8 and OS 9.1.2:
this Mac is capable of booting into OS 9, so OS 9 was fully usable for me including light Photoshop work with DIN A4 scans and such. I sometimed was too lazy to boot into OS 9, while I was in 10.2 and so I just opened classic and started OS 9 applications there. Apple Works, of course ran fluently. Master of Orion II, a space strategy game went fluently.
- ibook G4 1,33GHz, 1.5GB RAM, 320GB HDD (faster than the 10GB):
this Mac doesn't support booting into OS 9. I had 10.4.11 and OS 9.2.2 on it. I used CLassic there and Apple Works worked fine, but Master Of Orion II was very slow, moving the mouse resulted in the cursor jumping and it felt like the cursor was glued to the screen. The same with other Apps.
I have a PowerMac G4-AGP and several CPU Upgrades a Dual and Single 7447 1,8GHz among them, but I don't remember trying Classic with it. I currently have the 1,2GHz 7455 Upgrade by Sonnet installed, simply because it can be cooled passively (there were two generations, one passive and one actively cooled) so I can't comment further on the speed needed under OS 9 natively or in classic.