The beauty of SpeedGrader is the navigation you are afforded while using the feature. You should now be familiar with the image below: an assignment within SpeedGrader. Note that what is covered in this section is available anytime you are using SpeedGrader - yes, even in discussion forums.
See the red box that the arrow is pointing to? You may firstly use this to identify which student you are grading. Currently, the image above says "Test Student". This means that I am viewing the assignment that the Test Student has submitted. This text will change depending on which student you are grading.
Beside your students' name will be a left arrow and a right arrow. You may use this to navigate between your students. This means that once you are done grading, you can simply move on to the next student by either clicking left or right. By default, Canvas sorts your students alphabetically.
However, if you don't want to use the left and right arrow, that's fine too! What you can do is actually look for a specific student by clicking on your student's name. This will bring up a drop down list of all the students in your course. You can see that in my example course above, I have three students: Wesley Custer, myself, and the test student.
 If your student's name is greyed out, that means he or she has not submitted anything to the assignment or discussion forum, or has not completed the quiz.
 If your student's name has a green checkmark next to it, that means that you have graded their submitted assignment.
 If your student's name has an orange circle next to it, that means that they have submitted an assignment, replied to your discussion forum, or submitted a quiz, and you have not yet graded it.
These icons are a quick way to assess how many students have completed your activity, or how many activities you need to grade.