Why is using a sequence so much slower than using a list in this example

Question

Background: I have a sequence of contiguous, time-stamped data. The data-sequence has holes in it, some large, others just a single missing value.
Whenever the hole is j

Answer 1

Seq.skip constructs a new sequence. I think that is why your original approach is slow.

My first inclination is to use a sequence expression and Seq.pairwise. This is fast and easy to read.

let insertDummyValuesWhereASingleValueIsMissingSeq (timeBetweenContiguousValues : TimeSpan) (values : seq<(DateTime * float)>) =
  let sizeOfHolesToPatch = timeBetweenContiguousValues.Add timeBetweenContiguousValues // Only insert dummy-values when the gap is twice the normal
  seq {
    yield Seq.hd values
    for ((prevTime, _), ((currentTime, _) as next)) in Seq.pairwise values do
      let timeDiffBetweenPrevAndCurrentValue = currentTime.Subtract(prevTime)
      if timeDiffBetweenPrevAndCurrentValue = sizeOfHolesToPatch then
        let dummyValue = (prevTime.Add timeBetweenContiguousValues, 42.0) // 42 is chosen here for obvious reasons, making this comment superfluous
        yield dummyValue
      yield next
  }