Performance: Missing index for textgroup/location

i18nstrings_get_source() executes a query similar to this one:

SELECT s.*, i.type, i.oid, i.property  FROM {locales_source} s LEFT JOIN {i18n_strings} i ON s.lid = i.lid WHERE s.textgroup = S AND s.location = S

I.e., it filters the {locales_source} table on on the textgroup and location (type:oid:property) fields. However, core locale module doesn't define an index for those fields, which is causing massive slowdowns the larger the {locales_source} table grows. We noticed about 300-400 msec execution time *per query* (there can be many of them during a single request) on a very heavy {locales_source} table with ~45K entries.