Elsewhere in the comments section there were tips about an addon that tells if one of your addons is blocking e10s. Another useful tip was the dom.ipc.processCount setting. Before I cancelled my LastPass account I was using the 3.x version from them, which doesn't support e10s and crashed the browser when I forced it on. But still I was able to turn it on even though the browser was unusable.

I run this on Arch and Wayland, which might also add to the snappyness. What I kind of observed from all the comments was, that the OSX users were complaining the most about the slowness. Might be that the Linux version is faster then.