Change Stream fullDocument:updateLookup performance

I am using MongoDB Atlas and Node.js Driver. Sometimes I have noticed that making fullDocument:updateLookup slows down the real-time data fetching i.e. I am getting change/updated document lately instead of in real-time. What could cause such an issue? My document size is only 1~1.5KB max.

Our application is an IoT based application. So we are expecting to have a heavy update operation throughout the databases. The stream will sync real-time document with Redis cache.
Another implementation we may do is to go with the default configuration of the change stream. Then make a read operation with each real-time feed in order to fetch the latest full document. Will this better scale compare to fullDocument:updateLookup?

How well Change stream scale in a very high load scenario? Any Change stream scalability tips are most welcome !!!

Thanks in advance !!!