Slow Query

Hi Please could you advise what this slow query is when I am opening an order:

Query

SELECT DISTINCT meta_key
FROM wpng_postmeta
WHERE meta_key NOT BETWEEN '_'
AND '_z'
HAVING meta_key NOT LIKE '\\_%'
ORDER BY meta_key
LIMIT 30

Caller

meta_form
post_custom_meta_box
do_meta_boxes
require('wp-admin/edit-form-advanced.php')

Time 9s

Many thanks
Joe