To grade or not to grade

Hello, everyone!  This past week I spent almost the entirety of one of my snow days grading student work.  Granted, I had not realized how much of it had piled up, but still; it took me around five hours!  All of that time I spent assessing had me thinking about how much my teacher wants grades for little-taught subjects like science and social studies.  Two easy quiz grades often make up my students' entire semester grade for science.  The time I spent grading also had me thinking about how Metro encourages their teachers to grade and I wanted to pose some questions that were persisting in my mind.

First of all, part of the reason it took me so long to grade my students' work was that I was assessing them using a rubric that had several components to it.  I spent a great deal of time reading through their work, analyzing how much they actually understood about fractions, and trying to fit their responses into my rubric's scale.  At times, I felt that I was being overly punitive because some of the students who worked hard received a low score.  At first glance, anyone would think that their work was better than that of their peers.  But, based on the rubric I created, they scored poorly because they  had omitted important features.  I know that we have read many articles about the benefits of performance assessments, but do you have any ideas for how we could shorten grading time required?  Additionally, do you have any suggestions for how to remove the nitpicking tendency of rubrics without creating an overly-subjective grading system?

In the same vein, I have been frustrated throughout my placement by the scaling breakdown my teacher (and all of Metro) uses.  It seems perfectly fair when you first examine the breakdown:
F - 0%-59%
D - 60%-69%
C - 70%-79%
B - 80%-89%
A - 90% +

However, when the students are given short quizzes with ten or fewer questions, it soon becomes impossible to give fair grades with this percentage-letter correlation.  I gave a few quizzes with 5-8 questions on them because I did not want to take too much of my students time.  However, if a student missed one question on a five-question quiz, they are automatically reduced to a B.  If they miss two, they are downgraded to a D.  D and lower is failing.  So if they make two mistakes, they've failed.  How is that justified or fair?  And how can I justify giving my students longer and more emotionally taxing tests just so that they have a better chance of grading higher?

My last question about grading is based on something that my teacher told me.  She said that the special education teacher forbade them from giving failing grades to their students with learning disabilities.  However, if the students do not have much of a differentiated IEP (e.g. one of my students just requires extra time and things read aloud to her) and they are failing because of their learning disability, how do you justify that in your grading?  I already have a hard enough time giving ten-year-olds a failing grade, but how can you radically alter your grading parameters for special needs students without feeling like you are downgrading your other students?  I know that we need to provide equitable education, but I am a loss to see how that relates directly to grading and students' input of effort.