The sweep server under some conditions will evict entire b-trees, and for a large b-tree this can take an extended time (many minutes in some cases). While this is occurring any attempt to access the b-tree hangs.
Example: start mongod with --storageEngine wiredTiger --wiredTigerCacheSizeGB 9. Then populate a collection with 8 GB of collection and index data (using small documents as below probably exacerbates the issue because it makes the eviction slower due to the number of buffers that must be freed during the eviction):
Start mongostat to monitor the cache statistics while paging in the b-trees using this command:
While this is running you should observe the cache utilization climb to its limit, 80%.
Now wait a minute or two (so that the b-trees haven't been accessed in a while), then trigger eviction of the test.c collection and _id index by creating a new test.d collection (20 MB will do):
Soon after this (within a minute) the sweep server will begin evicting the test.c collection and _id index b-trees; you will see cache utilization drop from 80% to 0% over the course of some time (a couple minutes, somewhat longer on Windows than Linux). During this time accesses to test.c block.