On the nested <current/> element in a <clover-report> task.
Or, as attributes on <clover-html-report/>

numThreads - sets the number of threads to start when generating a HTML report. A value of 0 will disable multi-threading for report generation. Default is 2.

timeout - the amount of time to wait for report generation to finish before failing the build. This value must be an [Interval | Specifying+an+Interval]