I like the idea of FastGrade mode, but I got frustrated because I often want to see the code in the editor and waiting for the code to load takes time, especially when there are 100+ submissions to grade.
A technique I have started using is to start from the Code Review Dashboard. Then I right-click on a submission and open it in a new tab. I do this with 10-20 submissions. As I grade each submission, I close the tab and move on to the next, pre-loaded tab.
Maybe FastGrade mode could be improved to include an option for loading the code in the editor in a separate tab or for pre-loading 2 or 3 submissions in the queue. Waiting around for code to load adds up.