If the working set is yet in memory it will have to be fetched from disk.
The ideal performance for your primary is if all your working set can fit into 80% of your Wired Tiger cache. If that is not possible due to size limits consider trying to fit at least the indexes in those 80% as this will mean the disk Access be minimal and direct.
Example a 32GB server will be by default with 16GB Wired Tiger cache , 80% of the cache will be ~13GB …
MongoDB caches pages of WiredTiger the amount if documents that could fit there depands on the size of documents.