Using the glib event dispatcher, a simple application that only starts a timer with a timeout of 10 msec, takes about 10% of a cpu (core i7 4.3 GHz). See the project in the attachment.
The results were the same for versions 4.8.4 to 4.8.7. I haven't tested with older versions.
Using the unix event dispatcher (by setting "QT_NO_GLIB" environment variable), the cpu usage is almost negligible.
This performance issue does not exist on Qt 5.

Going through the source code, it would seem that there is a timeout rounding issue in the "timerSourcePrepareHelper" method in "qeventdispatcher_glib.cpp".
Indeed in Qt 5 the timeout is calculated as follows:

    timeout = (tv.tv_sec * 1000) + ((tv.tv_nsec + 999999) / 1000 / 1000);

while in Qt4 :

    timeout = (tv.tv_sec * 1000) + (tv.tv_usec / 1000);

With a similar rounding on microseconds, the performance issue disappears.
I attach a patch of the correction.