It’s difficult for me to say why your issue no longer reproduces. One guess I have is that your original order by query was not DESC. You can try seeing if removing DESC in the order by reproduces your slowdown.
instead of doing select * I specified specific fields and it got better aswell (6 seconds)
In older releases, we sometimes unnecessarily materialize projected non-filter columns. If you upgrade to 7.0, I expect that the select * query will be about as fast as your query that projects specific fields.
Regarding shard key tuning, I don’t think this is the right path to take for improving this query’s latency. I suggest sticking with keyless sharding for now. You may see a benefit on this specific query by placing game_id first in your clustered columnstore key. Alternatively, upgrading to 7.0 will also allow you to create a hash index on game_id, which I suggest experimenting with.