low performance of QNetworkAccessManager::get() while parallel http requests

I need parallel downloading in my APP.
1.I create One QNetworkAccessManager,
2.call QNetworkAccessManager::get() four times to sendout http request at the first.
3.when one request finishes, sendout another request using QTimer::singleShot().
I find out that, some times QNetworkAccessManager::get() need aout 1 second to return.
==========================================
06:40:44.168 [Qt Message] "[Remote Monitor:] " ////----///before
06:40:45.016 [Qt Message] "[Remote Monitor:] " ///////http://isa.nokia.com/ims/images/aa30356

06:40:55.347 [Qt Message] "[Remote Monitor:] " ////----///before
06:40:56.204 [Qt Message] "[Remote Monitor:] " ///////http://isa.nokia.com/ims/images/aa30356

06:41:00.160 [Qt Message] "[Remote Monitor:] " ////----///before
06:41:00.991 [Qt Message] "[Remote Monitor:] " ///////http://isa.nokia.com/ims/images/aa30356

06:41:07.451 [Qt Message] "[Remote Monitor:] " ////----///before
06:41:08.303 [Qt Message] "[Remote Monitor:] " ///////http://isa.nokia.com/ims/images/aa01875
=======================================

during this perioud, user operation can not be accepted by UI.

please refer to attach code and log.
Thanks!