FILTER clause for non-aggregate window functions

I need to find the first date value in widow which meets certain
conditions. In my case I use min() function with CASE like this:

SELECT min(CASE WHEN <conditions> THEN <date_field> end) OVER (PARTITION BY
.. ORDER BY <date_field> ROWS BETWEEN 1 FOLLOWING AND UNBOUNDED FOLLOWING)

But this is too slow mainly because of min() should read all tuples in
window, I guess. So I tried to use first_value() function like this:

SELECT first_value(<date_field>) FILTER(WHERE <conditions>) OVER (PARTITION
BY .. ORDER BY <date_field> ROWS BETWEEN 1 FOLLOWING AND UNBOUNDED
FOLLOWING)

But I got this error:

FILTER is not implemented for non-aggregate window functions

Why FILTER is not implemented for non-aggregate functions? Is there some
restrictions in PostgreSQL executor core or maybe this behavior will lead
to some conflicts? Is there a way to find needed value without scanning all
tuples in window? Any help would be greatly appreciated.

-- 
Andrey Zhidenkov