I did my graduate thesis on this, and discovered that timed tests CAN actually help students (albeit minimally), but only if they themselves are involved in charting their progress. I created a tracking sheet for my students when my school required timed tests. It had a place for the date, the test that student was on, their score that week, and whether they went up or down. Any time there was improvement, even if it was not a perfect score, was celebrated. When we stopped doing the timed tests, the kids actually missed them. They liked tracking their own progress. If you send me a private message, I will send you the tracking sheet I use. You can adapt it any way you like to fit your needs.