Ok, a 15 minute hack. It may be possible to make it look nicer.

And it looked better in TFA, so good job!

    SELECT device_id, 
        reduce(
           zip_with(
               array_sort(zip(array_agg(ts), array_agg(val))), 
               slice(array_sort(zip(array_agg(ts), array_agg(val))),2, 5000), 
           (a,b) -> abs(a.field1 - b.field1))
        , 0, (s, x) -> s + COALESCE(x,0), s -> s) as res
    FROM (
        VALUES 
        (1, 2, 1),
        (1, 3, 0),
        (1, 9, 3),
        (1, 3, 4),
        (2, 2, 1),
        (2, 8, 0),
        (3, 4, 3)
    ) AS t(device_id, val, ts)

GROUP BY device_id

    device_id res
    1        14
    3        0
    2        6