First - I do continue to use ML 9 Premiere on my desktop, but I've had a problem for some weeks now with the DVD/CD drive. It's partially solved, but not entirely, so I've been depending on my Latitude laptop for the occasional CD copy or other disc burning job. The question of updating ML 10 relates to the laptop, where I don't have ML 9. In the Dimension 3000 desktop, I edit video almost daily, and the age of the computer is not a problem. The response of my editing program (Adobe Premier Elements) is more than adequate. This computer has a Pentium 4 chip @ 2.83 GHz, which is about as fast as most computers have today. Single processor, yes, and just 1GB of RAM, but Premier Elements doesn't demand excessive resources.
As to the question of how well my computers are maintained -- the answer is very well. I do a disk cleanup often and check the need to defragment; there aren't excess programs running on startup. With both ML 9 and ML 10 in my desktop, ML 9 performs at top speed, just a few minutes to copy an audio CD, and reasonable time to burn or copy a DVD depending on length. The issue that arises is that in the same computer ML 10 takes 3-4 times as long to do anything. Even before I bought ML 10, I read user comments online saying it was much slower than 9. I bought ML 10 when it became available as a separate program from Roxio Creator, where the issue of bloat -- too many capabilities I don't need -- apparent. But even as a program devoted primarily to audio, 10 can't hold a candle to 9. For the cause, we should look to Roxio, not to my computers.