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I'm experiencing it too. I used to get horrible stuttering even if it wasn't active and, when it was, it was roughly 1 frame every 10 seconds. Couldn't type anything.
I found that right clicking and un-checking the Facebook Messenger option eliminates the lag, so it is obviously a problem with the sidebar, but as to what causes it and why it happens I do not know.
Bump
Been having the same issue as this, but additionally: I've noticed stuttering in regular browsing and especially when watching YouTube. If you right click on the Sidebar and uncheck "Facebook Messenger" it seems to help with the stuttering, but that still begs the question as to why this happens.