This article refers to some very firm research which shows that timed
testing is one of the reasons why students have difficulty with and hate
math. But notice that repeated extremely stressful situations have bad
effects. Small amounts of stress may be beneficial, but I do not know where
the threshold lies. Of course this relates to corporal punishment which is
also a stressful situation which sends the wrong message. But again, some
state standards imply that timed testing is needed. They don't bother to
look at research.
I think that the conventional method of teaching physics also produces
fairly high stress levels because most students find the subject to be
incomprehensible. Then of course there is the problem of tests with a time
limit which may be too short for some students. Other research has shown
that students who are LD benefit greatly from extended time, while "normal"
students do not. So giving anyone extended time is a good practice.