On my Mac, if I use the external editor, saving and running SLua goes well. I have noticed that often - not sure if it's always - editing directly in the viewer's script editor, I get about one character per two seconds, with beachballs almost continuously. I am on an M1 Mac with 16GB and SL is generally pretty OK.
At this point I'm not sure if it is always like that, as I usually use the external, but wanted to get the issue on the table. Thanks!