Are you sure your observer is using the oplog and not polling? If it’s polling it could take up to 10 seconds to trigger. You can use kadira to check this, but if you’re querying by _id you should be using the oplog (unless you’re also doing weird things with sort/limit). So long as your mongo db is a replica set (standalones don’t have oplogs by default)
While the overall approach could probably be improved, 5 seconds for observer triggering is excessive.