What are some research-based strategies to help students with pacing on standardized tests?

Some students take FOREVER to complete testing, for various reasons.  Three categories come to mind:  perfectionist students, slow processors, students with attention difficulties.  There must be some strategies that can be practiced/taught prior to testing to help students keep moving, because the extra time doesn't usually help them score any higher.  Even with accommodations such as separate setting, extended time and cuing these students will often take way too long on the first half of the test, and rush through the last part.  And I imagine there should be different strategies for the varying reasons that students are taking so long.  Thank you!