Hooligan may be right - it doesn't look terribly slow, but it sounds like it's laboring, some. Try giving it just a little bit of "play" and see if that helps.
It might be "dry," too - I'd oil it all up top and bottom, run it at full speed for a minute and let it sit, run it, let it sit, repeat... you can hear the machine free up if it's dry.
The next thing to check would be the button controller. I have a modern clamshell foot control on all my vintage machines and that helped them all. I wish Jenny was still in business at sew-classic - she sold all the cool stuff we need for these old gals and I don't know who has taken her place in the vintage world.
Hope the belt is the only culprit.
