I just tried the latest stable version on a MacBook Air from 2013 (with 10.11). I pasted 1 kB of text and it was blazing fast when editing outlines. When zoomed out to see the whole text it is lagging a bit but that is not hindering any work. So it is just a bit annoying. I have an idea on how to improve that.
Why it is so slow for you? Do you have any plugins installed? Those might slow things down.