Have you checked your internet speed? -->
http://www.speedtest.net/
Is the Time Capsule also your router?
What MacBook do you have?
Have you tried the HTML 5 beta version of YouTube or Click2Flash, which allow you to watch the H264 encoded video without the Flash overlay and might help?
Is the video loading completely or does it stop and does not load anymore?
What Mac OS X version and browser (version) are you using?