If one increases the replication factor of a keyspace and then do a repair, how will this affect the performance of the affected nodes? Could we risk the nodes being (more or less) unresponsive while repair is going on?  The nodes I am speaking of contains ~100gb of data.  
Also, some of the keyspaces I am considering increase the replication factor for contains Counter Column Families (has rf:1).  I think I have read that adding replication to counter cfs will affect performance negatively, is this correct?  

Cassandra version is 11.7.