This is very important: to access SpeedGrader in discussion forums, you will not be following the same steps as quizzes or assignments. Instead, on the discussion forum page, click on the three dots on the top right-hand corner of your prompt. A list of options will drop down, and that is where you will find SpeedGrader.
Clicking on it will open up SpeedGrader in a new tab.
Another important note is that you may not see SpeedGrader in this list. If you do not see SpeedGrader, that means that you have not turned on the "Graded" option when editing your discussion forum. Don't fret, however! This option can be turned on at any time, so you may do so even after your students have posted.
SpeedGrader collates every post that a student has posted in your discussion forum. You'll see that in the  text area, there is one post. However, if the student has posted more than once, you will see a list of all their replies in this text area. This means that, while you may lose some context because it pulls individual replies regardless of whom that student is replying to, it will give you an overall summary of that student's participation in your discussion forum.
You should be familiar with  and  - this is where you will assign them an overall grade, and give them overall comments on their participation.
Another difference in using SpeedGrader for discussion forums is that you do not have the ability to give individual comments on each reply, as you would have for quizzes, or add comments in assignments. This is because, based on best practices, Canvas encourages you to interact directly with your students in the discussion forum itself.