DECLARE @FindText VARCHAR(4000) = ''

SET NOCOUNT ON 

IF @FindText = ''

    -- сам по себе этот SELECT выполняется за 1 сек, а в составе ХП - за 10 сек
    SELECT TOP 100 caao.City_id, cnt = COUNT(DISTINCT caaa.aut_id)
    FROM core_art_ArtOrganisations caao WITH(NOLOCK)
    JOIN core_art_ArtAutors caaa WITH(NOLOCK) ON caao.art_id = caaa.art_id         
    WHERE caao.City_id IS NOT NULL        
    GROUP BY caao.City_id
    ORDER BY cnt desc, 1

ELSE

    SELECT TOP 100 City_id, cnt = COUNT(DISTINCT caaa.aut_id)
    FROM core_art_ArtIndex i WITH(NOLOCK) 
    JOIN core_art_ArtOrganisations caao WITH(NOLOCK) ON i.art_id = caao.art_id 
    JOIN core_art_ArtAutors caaa WITH(NOLOCK) ON caao.art_id = caaa.art_id         
    WHERE (
        CONTAINS(i.[art_title], @FindText) 
        OR CONTAINS(i.[art_abstract], @FindText) 
        OR CONTAINS(i.[art_keywords], @FindText)
        )
        AND City_id IS NOT NULL
    GROUP BY City_id
    ORDER BY cnt desc, 1

    SELECT TOP 100 caao.City_id, cnt = COUNT(DISTINCT caaa.aut_id)
    FROM core_art_ArtOrganisations caao WITH(NOLOCK)
    JOIN core_art_ArtAutors caaa WITH(NOLOCK) ON caao.art_id = caaa.art_id         
    WHERE caao.City_id IS NOT NULL        
    GROUP BY caao.City_id
    ORDER BY cnt desc, 1