Use below Ant task to pre-compile your reports and then use ".jasper"
files within your project. This will concentrated that 70-90% of report
compilation time in compile process, and also do a pre-check of report
validation. Remember to place jasper  reports library in Ant's classpath
and use only fillReport (no compileReport needed).

<taskdef name="jrc"
classname="net.sf.jasperreports.ant.JRAntCompileTask"/>

<target name="jasper">
<mkdir dir="${tmp.dir}"/>
<jrc destdir="${classes.dir}" srcdir="${src.dir}" keepjava="true"
compiler="net.sf.jasperreports.engine.design.JRJavacCompiler"
casesensitive="false" tempdir="${tmp.dir}"
followsymlinks="false">
<!--compiler="net.sf.jasperreports.engine.design.JRDefaultCompiler"-->
<!--compiler="net.sf.jasperreports.engine.design.JRJdk13Compiler"-->
<include name="**/*.jrxml"/>
<!-- under construction reports :) -->
<exclude name="**/NotYetReadyReport.jrxml"/>
</jrc>
</target>