It is possible to get the previous position using the ValueChanged event but as you mentioned it fires too quickly and implementing logic to calculate the previous position could cause performance issues.
private const int updateTime = 1;
currentVideoPosition = slider.Value;
(currentVideoPosition - previousVideoPosition < updateTime)
previousVideoPosition = currentVideoPosition;
And then when you click you will have the two values.
However there is another approach. Basically , RadSlider is mainly consisted of RepeatButton, Thumb, and second RepeatButton. You acntually see these three when you use RadSlider with no additional properties set.
So you could get these Repeat buttons from the template of RadSlider. Then, in their Click event handlers, get the RadSlider Value property and store it in field. After that the ValueChanged event will update the Slider's Value and then the Slider's MouseLeftButtonDown event will be triggered.
I am attaching you a sample project for your convenience.
Hope this information help.
the Telerik team