Union within View vs.Union of Views

Consider:

CREATE VIEW_X AS
SELECT <query A>
UNION ALL
SELECT <query B>
UNION ALL
SELECT <query C>;

versus

CREATE VIEW_A AS
SELECT <query A>;

CREATE VIEW_B AS
SELECT <query B>;

CREATE VIEW_C AS
SELECT <query B>;

where <query A>, <query B> and <query C> are each somewhat complex
with several joins, but utilizing different tables for each of A, B
and C.

Performance on

SELECT * from VIEW_X WHERE <conditions>;

was absolutely terrible. But performance on

SELECT * from VIEW_A WHERE <conditions>
UNION ALL
SELECT * from VIEW_B WHERE <conditions>
UNION ALL
SELECT * from VIEW_C WHERE <conditions>;

was nice and speedy, perhaps 100 times faster than the first.

If it's possible to consider this abstractly, is there any particular
reason why there is such a vast difference in performance? I would
guess that is has something to do with how the WHERE conditions are
applied to a view composed of a UNION of queries. Perhaps this is an
opportunity for improvement in the code. In the first case, it's as if
the server is doing the union on all rows (over 10 million altogether
in my case) without filtering, then applying the conditions to the
result. Maybe there is no better way.

I can post query plans if anyone is interested. I haven't really
learned how to make sense out of them myself yet.

For my purposes, I'm content to use the union of separate views in my
application, so if this doesn't pique anyone's interest, feel free to
ignore it.

Jeff