At the momemt, Speedgrader only allows one grade submission to be submitted at a time. This means if multiple graders are grading the same assignment or quiz but on different students at the same time, their grade submission data might not be saved properly. For example, one of the grades will be entered into the course Gradebook while the other is lost.
This is only mentioned in the Speedgrader documentation ( https://community.canvaslms.com/docs/DOC-12774-415255021 ):
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.
We received reports from instructors and TAs who are not aware of this limitation and have lost hours of work because of this.
Hopefully, Canvas will allow multiple grade submissions in the near future.
Since instructors and TAs would not normally check the Canvas documentation while grading, as a workaround to this problem, Canvas should display a message on top of the Speedgrader page to let the graders know if another grader is currently grading, and that only one grade submission can be done at a time.