Feature summary (what you would like to be able to do and where):
It is roughly the same level of expensiveness to check if multiple pages exist as checking a single page
I Propose new static method of mw.title, checkExists that takes an array of page names, and returns an associative array listing which ones exist and which dont.
This function should only increment the expensive parser count once + 1 times per X pages in a single call, since its much cheaper to do it this way (not sure what an appropriate value for X is. I think either 25 or 50 would be reasonable)
Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
When people need to look up if several pages exist at once, they can do so much more efficiently. This also allows to look up more pages without hitting the expensive func limit and not causing performance issues.
Benefits (why should this be implemented?):
Encourages bulk page existence lookups which should mildly improve page render time.