First question is always *have you updated your video drivers recently?* Beyond that make sure you don't have any other programs running in the background. Things like AIM and antivirus software are notorious for stealing CPU cycles and causing other programs to gitter. Beyond those things there is always the chance you've got a bunch of spyware or adware slowing your maching down. There are programs available to clear those out if you can't find any other causes for your slowdowns. There are a set of new graphics options in the latest patch you can try to help improve your frame rate as well. I'd suggest you just change one at a time to see if it improves your situation.

Like Monclova34 mentioned, please try turning down your anti-aliasing.

For the tutorials, you can also view them in full-screen by checking the "Full Screen" checkbox. This may help play them more smoothly.