MongoDB - Clarification on choosing randomly distributed shard key
While it's hard to say for certain without full context, I'm assuming its referring to the need to keep the working set in memory. 

A randomly distributed shard key would distribute the workload across an entire index, meaning that the entire index would need to be fit in memory to efficiently handle the workload. Performance would deteriorate once the size of this index on a shard grows larger than RAM, as the index on the shard key would need to be page faulting data in and out of memory. 

In contrast, a non-random shard key may have a "hot" subset that handles most of the working set. For example, consider a website where only newer "posts" by users are frequently accessed and older "posts" are rarely accessed. While the indexes on "posts" may be larger than available memory, only subsets of the indexes may need to fit in memory, reducing memory pressure and the potential of page faults.