Upfront / Fixer editor slow and/or not responding

Over the past two days the Upfront dashboard has become increasingly less responsive. I have to try many times to get into edit mode. Sometimes going back into the WP dashboard and then trying again helps, other times not.

Are there any WP or theme config.php setting that I can tweak in order to speed things up? It is getting to the point where I can not get much work done.