[cairo] Performance issue with get_text_extents

[Robert Prückl]

Dear all,

I am using gtkmm win32 2.22.0-2. cairomm version 1.8.4 is included there.
I want to draw 16 filled circles in a Gtk::DrawingArea which change
their diameters over time. For the screen update I am using the
Glib::signal_timeout() signal which calls the paint method with a
frequency of 30Hz.
I am encountering a performance issue while doing this.
The paint method of the class which paints the circles always calls
Cairo::Context::get_text_extents (not explicitely of course, and there
is no text rendering included) and this call eats about 95% of the
processing time of the whole paint function.
I discovered this by using 'Very Sleepy'
(http://www.codersnotes.com/sleepy) with the debug build of the
application.
On my laptop (dual core) one core is thereby working at 100% and there
is noticable stutter on the application screen.

Do you have an idea to get rid of that problem? Help is greatly appreciated!
best,
Robert