This is really cool!

I'm curious why CTEs don't seem to be getting used here?

  SELECT
  (
    SELECT
      (
        t.data,
        t.created_at
      )::jsonb_x_timestamp
    FROM
      telemetries t
    WHERE
      server_id = 'a0dcc312-0623-af60-4dc0-238301cc9bf8'
      AND data ? 'load_avg'
      AND t.created_at >= series.target
      AND t.created_at <= (series.target + INTERVAL '2592 seconds')
      LIMIT 1
  )
  FROM
  (
    SELECT
      target
    FROM
      GENERATE_SERIES(
        '2020-09-01 00:00:00'::timestamp,
        '2020-09-15 23:59:59'::timestamp,
        INTERVAL '2592 seconds'
      ) target
  ) series

I'm curious on the performance of:

  WITH series AS SELECT
    target
  FROM
  GENERATE_SERIES(
    '2020-09-01 00:00:00'::timestamp,
    '2020-09-15 23:59:59'::timestamp,
    INTERVAL '2592 seconds'
  ) target
  SELECT
  (
    t.data,
    t.created_at
  )::jsonb_x_timestamp
  FROM
    telemetries t
  WHERE
    server_id = 'a0dcc312-0623-af60-4dc0-238301cc9bf8'
    AND data ? 'load_avg'
    AND t.created_at >= series.target
    AND t.created_at <= (series.target + INTERVAL '2592 seconds')
  LIMIT 1

or even shuffling some of the other selects and chaining CTEs together. I'm even curious if you could somehow use a combination of CTEs to generate something more suitable for use with SYSTEM_ROWS (TIL!! will keep this in my pocket for the future!).

CTEs are never a guarantee for performance improvements (they make things worse at times) but I see them getting ignored a lot when at times they are a huge boost.

Edit: wow the formatting is really bad, hopefully the code stuff doesn't kill it for mobile users