Every row in a page like in the above URL apparently loads a separate image from
the server to display the histogram bar, e.g.

http://deadlock.netbeans.org/hudson/job/test-compilation/lastSuccessfulBuild/warningsResult/category.deprecation/statistics?object=package.org.netbeans.modules.visualweb.jsfsupport.container

For a large build, it therefore can take several minutes (and plenty of network
activity and server load) just to display a single bar chart.

This is silly. Consider using JavaScript and/or CSS to render bars using HTML.