fulltext search speed issue with SQL_CALC_FOUND_ROWS

[<wassuuuub]

I have some_table with 100,000 rows and with an
average of 500 words in some_column of each row. When
i do a fulltext search on this table using a query
such as the following, all of my results are under 0.1
seconds:

SELECT something
FROM some_table
WHERE MATCH (some_column) AGAINST ('some_search_term')
LIMIT 0,10

However, when i add the SQL_CALC_FOUND_ROWS keyword
like in the following query, some queries take longer
than 1 minute:

SELECT SQL_CALC_FOUND_ROWS something
FROM some_table
WHERE MATCH (some_column) AGAINST ('some_search_term')
LIMIT 0,10

How can there be a huge difference in speed if both
queries always return the exact same results?

Thanks,
TK

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