This is somewhat normal, but it depends on the environment and surroundings, as well as the rest of the computer (what you are running).
I would recommend that you change the buffer of YouTube, you can do this by right clicking on a Flash video and selecting "Settings". Then deselect "Enable Hardware Acceleration" on the first tab, and set the third tab scroll bar to 10MB.
The bottom line is that flash is very, very demanding. Hopefully those settings help a bit though.