The lack of compression has a huge effect on working set size, especially as the BSON representation doesn't really save space just CPU.
Mongo's behaviour when the working set exceed memory size used to be pretty terrible. (I moved on from the relevant project so I don't know how much of an issue it still is.) https://jira.mongodb.org/browse/SERVER-574 - essentialy what happens is that Mongo keeps trying to execute queries rather than throttling to let existing ones finish which causes even more thrashing and longer query times.
Reducing the memory consumption would curtail the onset of that.