Sounds like a side effect of relying on MMAP (and not doing compaction).
"Basically, the issue is that if data migrates to a new shard, there is no re-compaction yet in the old shard of the old collection. So there could be small empty spots throughout it which were migrated out, and if the objects
are small, there is no effective improvement in RAM caching immediately after the migration."
- Dwight Merriman (at the link in the parent).
"The kernel is able to swap/load 4k pages. For a page to be idle from the point of view of the kernel and its LRU algorithm, what is needed is that there are no memory accesses in the whole page for some time."
"Basically, the issue is that if data migrates to a new shard, there is no re-compaction yet in the old shard of the old collection. So there could be small empty spots throughout it which were migrated out, and if the objects are small, there is no effective improvement in RAM caching immediately after the migration." - Dwight Merriman (at the link in the parent).
"The kernel is able to swap/load 4k pages. For a page to be idle from the point of view of the kernel and its LRU algorithm, what is needed is that there are no memory accesses in the whole page for some time."
-antirez from http://antirez.com/post/what-is-wrong-with-2006-programming....