My years of teaching have led me to conclude that no student performs poorly because of limited ability. Students perform poorly because they do not work hard to learn the subject.
I have plenty of good students and even some great ones. But many students are not interested in the university. Some students must work at a job full-time and try to be a student in their spare time. I could not have done what they are trying to do--and many of them cannot do what they are trying to do. Here is my analogy. If a marathon runner has to interrupt his race in order to work for a few hours, he is not likely to win.
Other students are not serious. They just graduated from high school. They may not even realize that someone can study enough to learn what they are expected to learn. (I did not understand how much work was necessary when I first arrived at the university.)
Others may just be lazy.
The student with the lowest intelligence that took a course from me may have had real brain damage (I'll call him Trajan). He spoke slowly and had a dull look to him. He did not seem to "get it." It could be that at some time I taught students who were even less mentally able, but, if so, they kept quiet or never came to class, so I did not know how disadvantaged they were. Trajan had the lowest intelligence of any student that I knew.
Trajan would ask a question during class about a concept and I would explain and give an example. Trajan would still not understand, so I would break the concept into smaller parts and come up with another example. He still would not get it. I would continue in this vein. The other students in the course were frustrated because Trajan was so slow.
Trajan often visited me during office hours and asked questions about the material. He took every study-help that I gave and poured over it. I explained and re-explained and drew pictures and gave examples for hours.
Trajan made an "A." It was an honest "A"--I did not fudge the average because of his limitations. He worked so hard that he could answer questions on tests. I am not sure if he actually learned the concepts in the way that an "A" student should, but he did the work.
The real punch line of this story is that Trajan made the only "A" in his section of the course. I have never had a student work harder than Trajan. I have had one student work as hard, but she was brighter than Trajan.
Very few students work hard enough to get my attention, by asking lots of questions during office hours, but those students always make excellent grades. Some students think they are studying hard. The ones who really are studying hard always make the grade.
I do not know what happened to Trajan. Did he get his degree and put his persistence to work for an employer? Did he have to drop out for financial reasons? Did he encounter a field of courses that he could not work hard enough to pass? I still think about Trajan fondly and would like to know how he is doing.