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.
If you have any other suggestions for getting it going, I'd appreciate it.
One additional complaint I have about FF: It's hard to find out where to change the setting that I want. I constantly have to ddg for "How do I change $x" and I end up with 3 different answers for 3 different revisions. If anyone knows of a good resource for me to find this out, I would appreciate it.