Despite the apparent advantages of and recent advances in the use of visualization in engineering design and optimization, we have found little evidence in the engineering literature that assesses the impact of fast, graphical design interfaces on the efficiency and effectiveness of engineering design decisions or the design optimization process. In this paper we discuss two examples—the design of an I-beam and the design of a desk lamp—for which we have developed graphical and text-based design interfaces to assess the impact of having fast graphical feedback on design efficiency and effectiveness. Design efficiency is measured by recording the completion time for each design task, and design effectiveness is measured by calculating the error between each submitted design and the known optimal design. The impact of graphical feedback is examined by comparing user performance on the graphical and text-based design interfaces while the importance of rapid feedback is investigated by comparing user performance when response delays are introduced within each design interface. Experimental results indicate that users of graphical design interfaces perform better (i.e., have lower error and faster completion time) on average than those using text-based design interfaces, but these differences are not statistically significant. Likewise, we found that a response delay of 0.5 seconds increases error and task completion time, on average, but these increases are not always statistically significant. Trials using longer delays of 1.5 seconds did yield significant increases in task completion time. We also found that the perceived difficulty of the design task and using the graphical interface controls were inversely correlated with design effectiveness—designers who rated the task more difficult to solve or the graphical interface more difficult to use actually performed better than those who rated them easy. Finally, a significant “playing” effect was observed in our experiments: those who played video games more frequently or rated the slider bars and zoom controls easy to use took more time to complete the design tasks.