Re: Multiple DB’s and Advice on large BP sites
Here’s a couple links about improving BuddyPress and WordPress performance.
https://codex.buddypress.org/buddypress-site-administration/improving-performance/
http://www.prelovac.com/vladimir/wordpress-optimization-guide
I would test some of the tips in those docs before setting up multiple databases.
But see also:
http://wpmututorials.com/plugins/shardb/
https://wordpress.org/extend/plugins/shardb/