BuddyPress.org

I assume you’re at least on a decent-sized VPS / slice of some sort, or perhaps a dedicated server. A healthy, growing WPMU + BuddyPress install is not something that will do well in a shared environment or a small VPS / slice–at least not when you’re talking about 3000+ active members. Both of these applications under heavy use can require higher allocated memory limit for PHP. That is not unusual. Here are some other steps you can take to improve performance: https://codex.buddypress.org/getting-started/improving-performance/