Meta_query with relation 'OR' killing server CPU

function get_nb_offres( $typeOffre ) {

    global $type_bien_correspondances;
    $args = array(
        'post_type' => 'annonces',
        'meta_query' => array(
            'relation' => 'OR'
        )
    );

    foreach( $type_bien_correspondances[$typeOffre] as $typeBien ) {
        $args['meta_query'][] = array(
                'key' => 'type_de_bien',
                'value' => $typeBien
        );
    }

    $annonces = new WP_Query( $args );
    return $annonces->post_count;
}

My global $type_bien_correspondances is here :

$type_bien_correspondances = array(
    'Appartement' => array(
        'studios',
        'T1',
        'T2',
        'T3',
        'T4',
        'T5'
    ),
    'Immeuble' => array(
        'immeuble'
    ),
    'Programme neuf' => array(
        'programme neuf'
    ),
    'Maison, Villa' => array(
        'maison',
        'villa',
        utf8_encode( 'propriété' )
    ),
    'Fond de commerce' => array(
        'fond de commerce'
    ),
    'Terrain' => array(
        'terrain'
    )
);

And finally my call to the get_nb_offres() function is here :

// Within a function
return get_nb_offres( 'Appartement' );

My problem is when running this code my server CPU goes crazy and I can’t do anything but restart it. Commenting the ‘relation’ line make the code work but it’s not what I’m expecting.

I can still muddle through this problem running several WP_Query but I would prefer to understand where the bug comes from.

EDIT

Not really an answer but several clues to optimize my query.

Solutions Collecting From Web of "Meta_query with relation 'OR' killing server CPU"

Afaik, there’s no relation key for meta_query. Use compare instead and maybe also specify a type to speed things up with skipping types that are out of scope.

EDIT

$args = array(
    'post_type' => 'product',
    'meta_query' => array(
            // Used to set two meta keys IN RELATION TO EACH OTHER
        'relation' => 'OR',
        array(
            'key' => 'color',
            'value' => 'blue',
                    // Used to FILTER OUT/IN/LIKE THE VALUE NAME
            'compare' => 'NOT LIKE'
        ),
        array(
            'key' => 'price',
            'value' => array( 20, 100 ),
            'type' => 'numeric',
            'compare' => 'BETWEEN'
        )
    )
  );
 $query = new WP_Query( $args );