What would cause a UNION to take an excessive amount of time?

I have a statement that looks like:

QUERY A
UNION
QUERY B
ORDER BY SomeColumn

Query A and Query B each take a nominal amount of time to run, but when I put them in the UNION, it take 7-9 seconds which is unacceptable. In this case Query A returns 6 rows, Query B returns 7. So confusing...

I have absolutely no idea what would cause this, help would be much appreciated!

Here is an anonymized version of the script (I did not write this, so don't hate):

SELECT 
    'XXX' l_t, 
    s.p, 
    srst.c_b, 
    srd.a_d_f, 
    s.s_c, 
    sf.c_s, 
    srst.p_f, 
    s.s_r_i, 
    s.s_i, 
    s.s_d, 
    srd.m_s_d_l s_d, 
    srd.m_e_d_l e_d, 
    CASE WHEN (srs.s_s_t IS NOT NULL AND srs.s_s_t <> srs.s_e_t) 
        THEN 1 ELSE 0 END 's_f', 
    CASE WHEN (srs.c_s_t IS NOT NULL AND srs.c_s_t <> srs.c_e_t) 
        THEN 1 ELSE 0 END 'c_f', 
    r.r_i 
FROM 
    t_s_r_d srd 
    INNER JOIN t_s s WITH (NOLOCK) 
        ON s.s_i = srd.s_i 
    INNER JOIN i_s_r(12345) r 
    ON r.r_i = srd.r_i 
    INNER JOIN i_s_s_f() sf 
    ON (sf.s_i = srd.s_i)
    INNER JOIN t_s_r_s srst WITH (NOLOCK)
    ON (srst.s_i = srd.s_i AND srst.r_i = srd.r_i ) 
    LEFT OUTER JOIN t_s_r_s srs WITH (NOLOCK) 
    ON (srs.s_i = srd.s_i AND srs.r_i = srd.r_i) 
WHERE 
    srst.d_f = 0  
    AND ((srd.m_s_d_l >= someval AND srd.m_s_d_l < someotherval) 
        OR 
        (srd.m_s_d_l <= someval AND srd.m_e_d_l > someotherval)) 
    AND r.o_f = 0 
    AND r.i_f = 0 
    AND r.v_f = 1 
    AND r.g_i = 180 
    AND NOT EXISTS(SELECT * FROM t_c_r cdr WITH (NOLOCK) WHERE cdr.r_i = r.r_i)

UNION 

SELECT 
    'XXX' l_t, 
    s.p, 
    srst.c_b, 
    srd.a_d_f, 
    s.s_c, 
    sf.c_s, 
    srst.p_f, 
    s.s_r_i, 
    s.s_i, 
    s.s_d, 
    srd.m_s_d_l s_d, 
    srd.m_e_d_l e_d, 
    CASE WHEN (srs.s_s_t IS NOT NULL AND srs.s_s_t <> srs.s_e_t) 
        THEN 1 ELSE 0 END '开发者_StackOverflow社区s_f', 
    CASE WHEN (srs.c_s_t IS NOT NULL AND srs.c_s_t <> srs.c_e_t) 
        THEN 1 ELSE 0 END 'c_f', 
    c.c_i 
FROM 
    (t_s_r_d srd 
    INNER JOIN t_s s WITH (NOLOCK) 
        ON s.s_i = srd.s_i 
    INNER JOIN i_s_s_f() sf 
        ON (sf.s_i = srd.s_i)
    LEFT OUTER JOIN t_s_r_s srs WITH (NOLOCK) 
        ON (srs.s_i = srd.s_i AND srs.r_i = srd.r_i) 
    INNER JOIN t_s_r_s srst WITH (NOLOCK) 
        ON (srst.s_i = srd.s_i AND srst.r_i = srd.r_i)), 
    i_s_c(12345) c 
WHERE 
    srst.d_f = 0  
    AND ((srd.m_s_d_l >= someval AND srd.m_s_d_l < someotherval) 
        OR 
        (srd.m_s_d_l <= someval AND srd.m_e_d_l > someotherval)) 
    AND c.o_f = 0 
    AND c.i_f = 0 
    AND c.v_f = 1 
    AND c.g_i = 180 
    AND EXISTS(SELECT * FROM t_c_r cr WITH (NOLOCK) WHERE cr.r_i = srd.r_i and 
        cr.c_i = c.c_i)
ORDER BY s_d

---

Because UNION removes duplicates it has to sort the whole data set first...try to use UNION ALL instead....it is much faster since it doesn't need to remove the dups

---

Turns out it was the old style ansii style joins in conjunction with the newer style explicit joins that were causing the lengthy return. Quite interesting, if anyone could provide a reason why that would be fantastic!