[MPDev]

Saying you don't use any queries to generate a "live" feed is misleading if it requires you to set your RSS feeds to very short regeneration times; the RSS capability isn't meant to be run with such frequency (in other words, you are trying to use a part of vB in a way that it wasn't designed).

Cache Lifespan
By default, content generated by external.php will be cached for one hour. This setting alleviates the problem of abusive users repeatedly requesting updates from external.php. Some RSS applications do not respect the 60 minute refresh request that vBulletin sends with the information.

I saw one variation that appeared to use hooks, but that doesn't necessarily make it less intensive than what you have designed here; you've simply offloaded the processing requirement to another script.