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Speeding Up the Response

System response time has always been an important factor for designers to consider in human computer interaction. From a usability perspective, optimal response time is dependent on several contextual factors, including the task to be performed, user experience level, user expectations, and other related functions to be performed by the user. There are several studies that relate response time to performance. Butler (1983) reports a significant effect of system response time on user action time. The longer the system response time, the longer the user action time. Barber and Lucas (1983) report that shorter response times yield superior performance. Another study, Tobmaugh, et al. (1985), showed that errors increased in a reading task where participants had to read and answer questions, with more errors for slower display rates than for faster display rates.

It is generally accepted that on the Web, people are more satisfied with rapid response times. User surveys (GVU, 1996; Hamilton, 1997) have reported that slow Web response time was the number one problem listed by users. Slow response times include slow downloading and slow Internet connections. Of course, system response times and speed of downloading are not entirely under the control of the designer. Quality of phone lines, server speed, and CPU power are among the factors that ultimately determine response time. However, designers can anticipate speed problems and take preventive measures. In addition, context and expectations make a difference in visitors' reaction to slow times. Sears, et al. (1997) found that visitors who accessed documents containing text and graphics were much less tolerant with the process than when the document contained text only.

To maintain "reasonable" site response times (within ten seconds for modems ranging between 28Kbps and 56Kbps), designers should attend to the following guidelines.

  • Make sure there is feedback, indicating delay.

  • Keep the total page content to not more than 60KB of text and graphics.

  • Keep graphics less than 25KB.

  • For five or more graphics per page, keep each graphic at 10KB or less (see Figure 7.5).

    Figure 7.5Figure 7.5: CNNSI.com page with no subsites pull-down menu (Reprinted with permission.)

  • Resort to the use of thumbnails measuring 1″ x 2″ (see Figure 7.6).

    Figure 7.6Figures 7.6: Edmunds.com page with links (Reprinted with permission of Edmunds.com, Inc.)

  • Provide text-only options.

  • Avoid the use of animation if at all possible.

  • Avoid the use of multimedia and audio clips requiring the download of plug-ins.

  • Use progressive rendering, allowing text to download before graphics.

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