This happens when the GtkPerf test window is maximized on a 3840x2160 screen
(although the GtkPerf draw area remains much smaller):

GtkPerf 0.40

GtkEntry - time:  0,07
GtkComboBox - time:  0,52
GtkComboBoxEntry - time:  0,37
GtkSpinButton - time:  0,12
GtkProgressBar - time:  0,13
GtkToggleButton - time:  0,45
GtkCheckButton - time:  0,17
GtkRadioButton - time:  0,26
GtkTextView - Add text - time:  0,31
GtkTextView - Scroll - time:  0,00
GtkDrawingArea - Lines - time: 76,11
GtkDrawingArea - Circles - time: 10,23
GtkDrawingArea - Text - time:  0,39
GtkDrawingArea - Pixbufs - time:  0,53
 --- 
Total time: 89,65

The full test now needs almost 90 (!) seconds to finish, Lines test needs 76,11
(!) seconds now...