Meta query order

The site had 20K plus posts/pages/ect, freshly put live.

The hoster called. It had slow queries, 9 seconds on average. A lot, multiple per minute. The same query over and over again.
It was a pretty normal post table query. It had one part extra.
A meta query was to exclude all spot_closed posts. When I removed that it was quick. It didn’t show anything special.

The meta query was added as such:

$meta_query_spots = [
    'relation'     => 'OR',
        'key'     => 'spot_closed',
        'value'   => 1,
        'compare' => '!=',
        'key'     => 'spot_closed',
        'compare' => 'NOT EXISTS',

Check if the spot is closed, and if it a spot does not have a close status.
The fix, flip the checks. And bam query less then half a second.