Upfront is really slow. If I click for eg "undo" it starts

Upfront is really slow. If I click for eg "undo" it starts to render something for many minutes and my Browser (FF) gets black/dark and is not responding. The wordpresses I use are freshly installed. Same issue on every site. Where do I start to gt to the key? Is it normal that Upfront is a kind of slow way on genereal? I mean if I double click a text it takes a few second to be able to edit it.
Also this is noot related to "Fixer" theme. But your system in asking here forces me to put htere something. And Fixer is the only name I remember. (Upfront is not valid)