This would be my current suggestion: (
Edit: Note that this was written before a bunch of the responses here)
Speed Calculations
To calculate the speed of a character or object the basic formula used is v = d/t, where v is the speed of the object, d is the distance the object moved and t is the amount of time it took the character/object to move that much.
Usually the time it took for an object to move the distance is calculate by dividing the distance another object moved during that time, through the speed of that object. That requires knowing the speed of the other object, of course.
More details, such as more in depth explanations on how to figure out the distance and the time as well as examples on how to calculate speed, can be found on the following pages:
Slow Motion Calculations
Sometimes when calculating speed one might encounter scenes where time seems to move slowly from the perspective of a fast moving character. Fundamentally speed calculations can be performed in the same way as normal in such cases.
In other words one just has to figure out a timeframe through the movement of a reference object with known speed, measure how far the character moved during that timeframe and divide the distance through the length of the timeframe (v = d/t).
However, sometimes the time is not just slowed down, but appears outright frozen.
In this case, if a reference object with known speed can be visually confirmed to not have moved even 1 pixel (which requires the feat to happen in a visual media like a comic, movie or animation) one can figure out the timeframe, by saying that it most have been less than the timeframe that the object would have taken to move 1 pixel.
If that can not be confirmed different upper limits, with similar argumentation, can usually be confirmed. In cases with a moving camera/point of view it can be useful to compare the movement to an object that can be assumed to be static.
Especially for written feats another method can be relevant. In that one first wants to figure out how many times slower the time is from the real time and then figure out how fast a movement in that time really is, based on how fast it looks in the slowed down time.
So the formula would be (real speed of reference object / apparent speed of reference object) * apparent speed of object of intrest = real speed of object of interest
For example: If an object that really is 1000 m/s fast seems to move with only 10 m/s in slowed down time a character that seems to move with a human walking speed of
1.4 m/s in slowed time would move with (1000 m/s / 10 m/s) * 1.4 m/s = 140 m/s.
Useful values for the apparent speed of movement would be:
Sometimes even something like "time seems frozen" or that nothing moves is stated. Often this kind of statements are hyperboles. However, should that not be the case one may assume that the apparent speed of the reference object is less than or equal to 0.001 m/s.
Mind rule 7 regarding
Cinematic Time, whenever calculating feats involving slowed time.