Reliability and validity are integral concepts in assessment design. Test speededness, the influence of time constraints on test taker performance, is often an overlooked threat to reliability and validity, especially in classroom-based testing. The purpose of this study is to evaluate the degree of test speededness of classroom-based assessments that have been developed and tested for a curricular reading intervention study with third-grade gifted students. The degree of speededness for the assessments was calculated using a mixture Rasch model. The results indicate that a large proportion of students were influenced by test speededness for both posttests in both treatment groups. The implications of these results are discussed. (Contains 5 tables.)