Slowing Down Sped Up Videos
Python 2, 47 bytes

lambda m,r,l:m/reduce(lambda u,x:u/r+x-x/r,l,m)

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Takes timestamps sorted in descending order.
The idea is to compute the video duration as a polynomial in the inverse speedup rate 1/r using Horner's method with coefficients given by the sorted timestamps. This avoids needing to explicitly take the differences of consecutive timestamps. We then divide the duration of the original video by the resulting duration to get the desired slowdown factor.
53 bytes

f=lambda m,r,l:l==[]or r/(l.pop()*(r-1)/m+1/f(m,r,l))

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An attempt to write the function fully recursively. While the new duration itself has a clean recursive expression, we want to get the slowdown factor which divides the original duration by the new duration, and this is messier to express recursively.