If you haven't seen this before, it may help.

http://www.mathsolutions.com/documents/9781935099031_message18.pdf

Especially this part:
Quote
When I tell this story, sometimes teachers share other
approaches to helping students develop speed in computation.
For example, if a teacher or school chooses to
include speed as a mathematics priority, the teacher
might offer students the option to work toward their
best time during a six- or nine-week grading period,
designating a specified block of time one day a week for
this type of work. When a student is ready to try to
improve his previous time, he can request to take the
test. Students record their starting and ending times,
doing their best to complete the test quickly and beat
their best time, but always finishing and competing
against only themselves. This type of self-administered
assessment carries much less stress and allows each student
to complete the test without competing against
other students.