The query used to generate Special:MostLinkedPages takes a very long time to run when the pagelinks table is large. (On my system, with 3 million rows, the query takes over 30 seconds, resulting in 502s to our load balancer.) Adding an index on (pl_namespace, pl_title) cuts that by around 60% (10 seconds on my system). It was suggested on IRC that this would also benefit Special:Whatlinkshere.
Version: 1.23.0
Severity: enhancement
I believe Special:MostLinkedPages is disabled on Wikimedia wikis due to performance considerations. It would be nice to at least get an estimate of the cost involved (additional indices, space on disk, etc.) in making it work well enough for a large site. Of course the "pagelinks" tables are probably some of the largest.
It seems in the meantime such an index was added
INDEX pl_namespace (pl_namespace, pl_title, pl_from)
Not sure if that helps for the MostLinkedPages, but I would assume there is no more to optimise at the moment and see this task as fixed.