Yes. ORDER BY ASC is very fast when you have a matching index, because the sort is avoided.
For ORDER BY DESC, you can convert it to an ORDER BY ASC as follows.
CREATE INDEX ON b( x );
SELECT ... ORDER BY x ASC;
Convert to these (for example if x is a number).
CREATE INDEX ON b( -x );
SELECT ... ORDER BY -x DESC;
This is a workaround. We are adding fast support for DESC in the next major release.