Q: My PowerBook G4 12" (Late 2004) is really slow. Why?

Kernel panics are usually a sign something is more seriously wrong.  Either the directory or the hardware is a problem.   The directory can also be confused for a dying hard drive.

I was plagued with kernel panics and sluggish performance on my 12" PowerBook, so I backed up the computer, wiped the drive and reinstalled the original OS. The machine came back to life snappy as ****, but the kernel panics persisted. I read somewhere that the AirPort card can cause kernel panics, so I reseated it, but finally removed it. Problem solved. Now it's on Ethernet.

I don't use this machine for Internet access because anything Adobe Flash grinds it to a halt. I use it in my shop office with a 20" Cinema Display to run VectorWorks, graphics apps, Office, and tons of other still-usefull PowerPC and OS 9 apps, as well as chat, watch movies, play games, write emails, play music, scan and restore old family photos, etc.

My experience with the PowerBook getting slower and slower with every software update convinced me to never upgrade anything, ever again, until I have to be dragged kicking and screaming. Not even security updates – I am willing to risk the very low probability of a security exploit for having a snappy fast machine. Plus, it seems that with every update, something you love goes away, and something you hate gets added. My MacBook Air is still running Snow Leopard (and at least a dozen perfectly good PowerPC apps). I used a PowerMac 6100/66 with OS 7 and a DOS card running Windows 95 for eleven years. I drove my last car for twenty years. I still use my 1st-generation iPhone. As long as I have my stuff backed up or have a Plan B, I'll take something to its death bed. Or, in the case of my nearly 20-year old PowerMac, give it away to a neighbor who is still using it.

I don't know why I didn't give you this tip before, but you should really be aiming towards data recovery first:

http://discussions.apple.com/docs/DOC-1689

And then seeing if you can repair the directory (also in that article).

If the data is not important, trying to repair the directory is the first step.

If that fails, then getting a new hard drive is probably your best course of action, and installing a bare essential operating system on it.  If it panics, then you know some hardware such as RAM is bad with your system.  Only use RAM designated for your Mac model and age of Mac.  Do not buy RAM based on timing or chip type.