Flash Player 10 is very much broken on older Mac computers as of March 2009.
I know it sounds too old but my g4 400mhz is very fast because the install of 10.4.11 is very clean. Flash 9 was working great with firefox3 and Safari3, both with all updates.
Then I updated to flash 10 and the problems were so obvious. Websites that used flash for plain animation would crawl, and any video would skip.
Here is a good test video of wall-e ( uses youtube )
http://www.youtube.com/watch?v=QHH3iSeDBLo
On flash 9 there are is almost no frame skip. Looks beautiful.
On flash 10 you get a new frame only once in a while, even audio skips.
I found tools on Adobe's website to remove flash 10.
I rebooted and installed flash 9 ( same tool page )
I rebooted and things work awesome.
http://kb.adobe.com/selfservice/viewContent.do?externalId=tn_14266
read the Adobe help tips, they help. I used flash 9r159, the easy to use installer was in a zip file called mac "UB" (whatever that means )
test your web browser and flash plugin test here
http://www.adobe.com/shockwave/welcome/
This is the perfect example of bloatware that harms the environment and creates e-waste. My mac g4 with OS 10.4 is a showcase of efficiency. All websites work and even the latest version of iTunes runs ok. But any normal person would not make the connection that flash 10 was the hog that made this computer seem ready for the dump. Most normal people would not be able to remove flash 10 and find and re-install flash 9. I'll be contacting adobe. If they are going to write such poor code their installer should halt and say that PPC macs can't support the flash 10 version. The flash 10 installer should leave a PPC mac untouched and working well with flash 9. And website developers should know to avoid flash 10 because they are forcing visitors to go away or go out and buy new computers in a world crazy with consumption.
I'll be contacting adobe. They really missed a major bug or they really don't care about the earth under their kids feet. I used to work at Macromedia testing plugins like this. Heck I even tested the first ever plugin for web browsers, netscape 2.0 and director ( shockwave ) 4.0.
I reported the bug. You can register a new username and then use the menu to "watch" the bug. My guess is that they will delay, deny or diminish this very major bug.
http://bugs.adobe.com/jira/browse/FP-1647