Computing Matching Data Points from Fuzzy Timestamps in Postgresql

In your second approach, with the self join, you could remove duplicates using row_number(),

Partition by l.charttime, order by the time difference and filter for row_number = 1.

I think performance will be horrible, however. Because of the cartesian join this will be an O(size(series 1) x size(series 2)) operation.

Having both l.charttime and r.charttime inside functions may also be causing trouble. Try refactoring to (in pseudo code)

    r.charttime < l.charttime + 3600
and r.charttime > l.charttime - 3600

.. and see how the query plan looks. I presume there's an index on charttime. Without one no approach will be fast. Indeed, two partial indexes, one on Series 1 and one on Series 2 may be even better.