edX has started to offer timed exams.
This feature allows course teams to assign a set amount of time to all the problems within a subsection. If a learner goes over the time lime, she can no longer submit answers to problems and does not receive points.
An additional interesting feature is that course teams can grant individual learners extra time to complete a timed exam.
The way it works is explained in this section of “Building and Running an edX Course”.