set lastactivity on a big board

[OcR Envy]

I'm running into some slow queries with setting lastactivity on a big board that I develop for(1,285 concurrent users at the time of this post). This is a default vBulletin installation with very little modification.

Examples:








I've never noticed a problem in any of the smaller boards I run but this one seems to be locking up during peak times and it's all do to these queries. So I took a look and found in core_class.php on line 3,640:

// if this line is removed (say to be replaced by a cron job, you will need to change all of the 'online'
// status indicators as they use $userinfo['lastactivity'] to determine if a user is online which relies
// on this to be updated in real time.

My first thought is just set a cache or cookie/timeout to only update lastactivity say once every 10 minutes. However after reading that I'm wondering how often does 'real time' mean. I would assume it's based on what our cookie_timeout is set to, yes?

Any suggestions on what the problem is, how to resolve it, or any work around examples would be greatly appreciated.