The refresh rate of the plot in the oscilloscope example depends on a timer with an interval of 10ms ( of course it is not really 10ms as QTimer is not an accurate system timer ). The timer interval checks if the curve has changed and draws new samples only. The sample rate and how to gather samples is completely unrelated to this.

For 1000 samples every 15ms I would expect a refresh rate of 10ms as being too short - depends on your system. The fastest Qt environment for this type of application should be Qt 4.8 with the X11 graphicssystem ( = native ). With Qt 5 or Qt 4 on Windows ( or Qt 4.8 on X11 not using the native graphicssystem ) you will end up with a pure software renderer.

Uwe