Drop collection becomes slow after system runs for few weeks

Our system generates relatively large amount of DBs (few hundreds), keeps them for a while and drop the oldest when needed to create new ones.

We see that after running for several weeks, the drop operation become slow and takes large amount of CPU.

We do the DB Drop by dropping all collections first and then the DB itself. It seems that the collection drop becomes ~x3 times slower and so does the CPU consumption.

Restarting mongod solves the problem (for the next few weeks).