core_user_get_users web service SQL queries do not consider indexes

Code is in user/externallib.php, function get_users

For example, when searching on the username, the generated SQL is something like...

SELECT * FROM mdl_user WHERE deleted = 0 AND username = 'fred' ORDER BY id ASC;

This is not indexed and (again, for example) on my site takes nearly a minute. This query should also use the default mnet id to take advantage of the index. 

On large sites, this web service can be very slow.