[Suggestion] add index referred to wp_statistics_visitor

I noticed the statistics dashboard loading for several minutes if not used for some time. Since – at least here – the table seems to be locked a lot during this time new visitors may also encounter extensive delays while an admin is loading the statistics dashboard. In MySQL i see a lot of unindexed queries for wp_statistics_visitor.referred – maybe worth considering an additional index – (yes, this slows down the overall speed a bit but prevent this hickups…)
ALTER TABLE wp_statistics_visitor ADD INDEX(referred);

https://wordpress.org/plugins/wp-statistics/