You can use the hasExpired() function to start your processing only after a specified amount of milliseconds has been expired.
Please refer to the QT documentation for more details:
http://qt-project.org/doc/qt-4.8/qelapsedtimer.html#hasExpired[
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There you will find the following example:
void executeOperationsForTime(int ms)
{
QElapsedTimer timer;
timer.start();
while (!timer.hasExpired(ms))
slowOperation1();
}