I'd go check if you're being DDoSed or see where your traffic is originating from; tools like
Cloudflare limit abusive bot traffic and clearly identify if you're being attacked and what countries to ban to alleviate abusive traffic.
UBB.threads uses the MySQL database as a storage engine, every user accessing the forum will have multiple hits to your database while they're visiting your site; the system should clear these hits automatically; during times of traffic increases (especially on large forums with lots of bot activity) you'll continue to see large traffic hits to the database, this is where services like
Cloudflare (which are free) come in handy to mitigate attacks.
