I can't say I've noticed but I block flash most of the time and use the awesome ClickToflash ( http://rentzsch.github.com/clicktoflash/ ) so I can use the H.264 option on YouTube whenever possible. I far prefer not having to deal with Flash unless I know I need to for an actual reason.
I'm not sure running older versions of Flash is a great idea... As has been mentioned in this thread it's had a lot of security vulnerabilities, which is part of why there's been so many updates to it, not just to make it perform worse on older machines
If you have trouble playing a file and it's possible to, another option would be downloading the .flv file and converting to mp4 in something like Handbrake.
In Safari it is very easy to download most flv files, just go to Window>Activity and look for the file ending in .flv, double click it and it'll download, then you can re-name it and convert it to MP4, or try playing it in VLC.
I'm not sure running older versions of Flash is a great idea... As has been mentioned in this thread it's had a lot of security vulnerabilities, which is part of why there's been so many updates to it, not just to make it perform worse on older machines
If you have trouble playing a file and it's possible to, another option would be downloading the .flv file and converting to mp4 in something like Handbrake.
In Safari it is very easy to download most flv files, just go to Window>Activity and look for the file ending in .flv, double click it and it'll download, then you can re-name it and convert it to MP4, or try playing it in VLC.