Performance elasticsearch with 2 node cluster with new and old machine

Hi Community,

I setup at the moment a new ELK cluster. Node 1 is already running on a new Machine with 64 GB Ram and 12 CPU core on a Raid 1 (System) and Raid 10 (SSD's, Data). This system is running with one shard.

Now I want add a second node to the cluster as a replica. Main function for backup not for performance issue.
Because the Hardware is running, but is a old machine, with 12 GB RAM , 2 Core on Raid 1 (System) and Raid 10 (Data).

Now i read that a replica is also used for requests balancing. So now my question.
When I attach the Node 2 as Replica with this slow performance hardware to the cluster. is that slowing down the complete cluster?

Is there a possiblity that the replica will only be active when the master is not reachable?

Thanks for Help