SpeedGrader documentation ( https://community.canvaslms.com/docs/DOC-12774-415255021 ) states that only one grader from a course can work in SpeedGrader at a time. Users report that not all grades and comments are saved when multiple graders from the same course are using SpeedGrader simultaneously. This is a known limitation in SpeedGrader and we would like to see this updated. This is particularly important for large courses.
SpeedGrader is generally designed for one instructor role to grade submissions at a time. Because of how SpeedGrader data is loaded and stored in the browser, multiple users should not grade assignments at the same time since each grader cannot view the most recent information for a submission. Updated grades also affect the Gradebook.