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

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 just a single missing value, I want to patch the holes using a dummy-value (larger holes will be ignored).

I would like to use lazy generation of the patched sequence, and I am thus using Seq.unfold.

I have made two versions of the method to patch the holes in the data.

The first consumes the sequence of data with holes in it and produces the patched sequence. This is what i want, but the methods runs horribly slow when the number of elements in the input sequence rises above 1000, and it gets progressively worse the more elements the input sequence contains.

The second method consumes a list of the data with holes and produces the patched sequence and it runs fast. This is however not what I want, since this forces the instantiation of the entire input-list in memory.

I would like to use the (sequence -> sequence) method rather than the (list -> sequence) method, to avoid having the entire input-list in memory at the same time.

Questions:

1) Why is the first method so slow (getting progressively worse with larger input lists) (I am suspecting that it has to do with repeatedly creating new sequences with Seq.skip 1, but I am not sure)

2) How can I make the patching of holes in the data fast, while using an input sequence rather than an input list?

The code:

open System

// Method 1 (Slow)
let insertDummyValuesWhereASingleValueIsMissing1 (timeBetweenContiguousValues : TimeSpan) (values : seq<(DateTime * float)>) =
    let sizeOfHolesToPatch = timeBetweenContiguousValues.Add timeBetweenContiguousValues // Only insert dummy-values when the gap is twice the normal
    (None, values) |> Seq.unfold (fun