Correlation Of Playback Speed & Duration

Recently youtube added speed control on video playback on mobile app. Previously it was only for desktop browsers. Now, if I increase the speed to 1.25 then how much time I would save while viewing particular video? Would it take 25% less time than original video? 

Interpreting it like that way is totally wrong way. Without going too much into technical terms, let's name 'content' for the whatever video has for it's entire duration. One thing is sure with playback speed of increased the duration for which we would view is reduced. The (oversimplified) formula in this case should be,
Speed  = Content / Time.....(1)

Speed 1 be the normal speed & T1 be normal duration. Also let Speed 2 be the increased speed say x1.25 & obviously T2 will be reduced duration in the case.

Speed 2 = 1.25 x Speed 1.....(2) 

Let's put both case in the formula (1),

Speed 1 = Content / T1

T1 = Content / Speed 1 ....(3)

Speed 2 = Content / T2

T2 = Content / Speed 2 ....(4)

Taking ratio of (3) & (4),

T1/T2 = Speed 2 / Speed 1.

Remember the content is not changed in the case.

T1/T2 = (1.25 X Speed 1) / Speed 1

T1/T2 = 1.25

T1 = 1.25 x T2

T2 = (1/1.25) x T1 

Hence, 

T2 = 0.8 x T1

This means, with increased speed to x1.25 it takes 20% less time than normal video playback not 25%.

Similarly, if we increase the playback speed to x1.5 then,

T2 = (1/1.5) x T1

T2 = 0.67 X T1

In this case, video will take 33% not 50% less time than normal playback speed.

On the similar note, we can find the new duration if playback speed is reduced. 

So the simple formula, to find the new duration is 

T2 = (1/x)T1 ........where 'x' is the factor by which playback speed altered.  

This should be applicable to any video or audio playback & not just youtube.