Mongos server exlpanation

Hello guys. I have a question. AS i understand, mongos works as load balancer between shards.
And, it query data from shard in parrallel in sharded cluster. So, my question is:
“When i add new shards to my sharded cluster mongos will works slowly?”
I suppose that collecting data from multiple shard will working slow when i’m not using shard key in query.
Am i right?

The mongos it self won’t be slow (unless the resources can’t handle the traffic), but when you add more shards it is important to have a good shard key, and to query using it. This will avoid collection scans and scatter gather queries which can slow down query performance.

@tapiocaPENGUIN tganks for reply.

But what if i want to scan all collection? Shardinf will scale query performance with scater gather linearly or not? If i have 100 shards mongos will do more work and will be slowly than if i will have 3 shards? So, to be clear:
Does adding new shards with scater gather queries improve performance? Or it will be pure because of mongod will do more work with new shards?
My all system works around scater gather queries. Shoud shard cluster improve performance for me?