MacOS – Free (if possible) Video Converter

I experienced a similar problem with a 2012 MacBook Air with two users logged in. Both users logged in, stuttering video via AirPlay, OS X 10.8, 8GB RAM, plenty of room on the SSD. When using an iPad or iPhone on the same wireless network and AirPlay, no stuttering.

Logged out the second account, no stuttering.

I haven't solved any of your other issues, but did confirm the concurrent login problem.

Seems like it is a pretty common issue. Have you tried clearing your cache and browser data? Also do you get the same error with different browsers? Try Safari and see if you get the same error.