This might help you when your flash plays too fast and you have a strong video card like an ATI or Nvidia.
First answer this:
Do you have a video card with an HDMI port on it?
If you aren't sure you can look for this: (FWI its close to the same size as a VGA port)
If you answered yes, follow these steps and hopefully it will solve the
Youtube Flash issue.
Kicker Menu > Computer > System Settings > Multimedia
Select "Phonon" on the left.
With Phonon selected, click on "Audio Output" at the top.
Notice the "(HDMI)" next to one of the devices on the right lol?
Simply click the HDMI device and then click "Defer" a few times, this should solve your Flash problems permanently.
(GNOME 3 Instructions)
system settings > Sound > Hardware Tab
Select one of the devices, then look down a little further at "Profile"
If it shows (HDMI) in the profile box, click the dropdown menu and select "Off"
The reason why this happens is HDMI is both video AND audio. If both are being processed through a video card, theres nothing slowing it down for normal playback.
Will add instructions for these later when they are in high demand: