MongoDB Fragmentation affect the Performance in WiredTiger
It will be less efficient to fetch data if you have a lot of space available for reuse. However, that space could be reused if you continue to insert or update documents in the collection.

If you've dropped a significant amount of data (which sounds like ~300GB in one of your collections) you can reclaim space by either:


re-syncing members of a replica set (recommended if you want to reclaim space from all collections)
running the compact command (important caveat: this will block all operations for the database the operation is running on)


For either approach I would use rolling maintenance to maintain availability.

A resync would be the most effective option. The compact command is riskier to run as a replica set member may become stale while running compact offline (and require resync); this operation may also not be able to free as much disk space as a resync and needs to be run per collection.