Extra time when time is of the essence (U.S.) In the U.S. context, I am wondering how colleges deal with the accommodation of students with disabilities when testing something for which "time is of the essence". 

Consider, for example, a sight-reading exam at a music school. Surely, as much as dyslexia seriously impairs reading text, there must exist an equivalent condition impairing some people's ability to read music. So I would expect that under current policies, a student who can demonstrate a serious "music reading disability" could ask for accommodation in the form of extra time on a sight-reading exam (at least if the exam is one where students are given some time with the new sheet before they have to perform it live)?

The issue, obviously, is that time is of the essence in a sight-reading exam. What is being tested is precisely whether the student is able to quickly (sometimes even "on the spot") read a new sheet of music. 

Of course, this extends beyond the particular example of sight-reading. A school that trains mechanics may specifically want to test whether its students are able to service a car in a given amount of time. A school that trains chemists may want to test whether its students can perform a given experiment in a given amount of time as well. And so on and so forth. 

These situations seem hard to reconcile with requests for extra-time from students with disabilities. My questions are:


Do current accommodation policies recognize a "special kind of tests" where "time is of the essence" and requests for extra time from students with disabilities can legally be turned down?
If so, what prevents teachers from deciding that time is of the essence in all (or most) of their tests? In answers to other questions about extra time, we often read that "tests should test knowledge and understanding, not speed". What if a school decided it wanted to grant degrees to student who not only "understand" some material, but are also "quick on their feet" and able to apply this material to solve problems (or produce an artistic performance in the musical example above) in a timely manner? Could the school go down that road without contravening existing anti-discrimination regulations and risk losing all its public funding?
Do current accommodation policies recognize a "special kind of tests"
  where "time is of the essence" and requests for extra time from
  students with disabilities can legally be turned down?


No. 

For example, where I am, instructors receive a form simply saying, "Student X has a right to double time on exams". There is no sub-categorization made to that right, and no channel for instructors to pick or choose whether to honor it.