Why does coalescing make this query quicker?

I have the following query

SELECT SQL_NO_CACHE
    `table1`.*
FROM 
    `table1`
LEFT JOIN `table2` ON table2.id = table1.table2_id
WHERE 
    (`table2`.`date_assigned` >= '2015-06-21')
    AND (
        `attempt_1_date`IS NOT NULL
        OR `attempt_2_date`IS NOT NULL
        OR `attempt_3_date`IS NOT NULL
        )
    AND (
        `attempt_1_date` >= '2015-08-16'
        OR `attempt_2_date` >= '2015-08-16'
        OR `attempt_3_date` >= '2015-08-16'
        )
    AND (
        `callback_date`IS NULL
        AND `callback_by_account_id`IS NULL
        AND `callback_result`IS NULL
        )
    AND (
        (
            `attempt_1_result` NOT IN ('complete,incorrect,decline,prospecting')
            OR `attempt_1_result` IS NULL
            )
        AND (
            `attempt_2_result` NOT IN ('complete,incorrect,decline,prospecting')
            OR `attempt_2_result` IS NULL
            )
        AND (
            `attempt_3_result` NOT IN ('complete,incorrect,decline,prospecting')
            OR `attempt_3_result` IS NULL
            )
        )

    #AND (`table2`.`date_completed` IS NULL)
    AND (COALESCE(`table2`.`date_completed`, '') = '')

If I coalesce the date_completed field first, my Database program says the query comes back in 0.000 seconds, yet if I used the above (commented out) line, just checking IS NULL, it takes just over 10 seconds. Both only return the same 5 results.

table1 has 24 columns, and ~171,000 rows, and table2 has 172 columns*, and 1.7 million rows. In 1.4 million of those rows, date_completed is null.

If you need any more information, just let me know.

Few more details I was advised to include:

MySQL version: 5.5.41-0ubuntu0.14.04.1-log

Explain outputs:

Using COALESCE

`id`, `select_type`, `table`, `type`, `possible_keys`, `key`, `key_len`, `ref`, `rows`, `Extra`
1, 'SIMPLE', 'table1', 'index_merge', 'table2_id,attempt_1_date,attempt_1_result,attempt_2_date,attempt_2_result,attempt_3_date,attempt_3_result', 'attempt_1_date,attempt_2_date,attempt_3_date', '9,9,9', NULL, 8, 'Using sort_union(attempt_1_date,attempt_2_date,attempt_3_date); Using where'
1, 'SIMPLE', 'table2', 'eq_ref', 'PRIMARY,ind_table2_date_assigned', 'PRIMARY', '4', 'database.table1.table2_id', 1, 'Using where'

Using IS NULL

`id`, `select_type`, `table`, `type`, `possible_keys`, `key`, `key_len`, `ref`, `rows`, `Extra`
1, 'SIMPLE', 'table2', 'ref', 'PRIMARY,ind_table2_date_completed,ind_table2_date_assigned', 'ind_table2_date_completed', '9', 'const', 10, 'Using where'
1, 'SIMPLE', 'table1', 'ref', 'table2_id,attempt_1_date,attempt_1_result,attempt_2_date,attempt_2_result,attempt_3_date,attempt_3_result', 'table2_id', '5', 'database.table2.id', 1, 'Using where'

Few more notes.

• date_completed is type DATETIME
• There are no entries where date_completed = ”

_{^{*120 of those rows relate to 60 questions, in the format question_x_score, and question_x_value}}

`attempt_1_result` NOT IN ('complete,incorrect,decline,prospecting')
        OR `attempt_1_result` IS NULL

AND `table2`.`date_assigned` >= '2015-06-21'
AND `table2`.`date_completed` IS NULL