If it is just laggy and slow during playback in the time line, Don't worry about it, you can preview render that section and it should play better. You can also drop the playback resolution down (will not affect your final video) so you can see it without the lag.
By the way, what is the source of the screen capture? Is it youtube or some other online service? I find that some youtube videos lag on my screen, stop and start, etc. That is probably a function of my RAM, CPU, and internet connection.
If it plays back fine in the MEP timeline and it only happens after you export and post on youtube, let me know.