If you find yourself planning to add more than 8-10 replication feeds in a Swarm cluster, you should probably reconsider your design. While Swarm can support a much larger number of feeds, the performance impact will become significant as the number of feeds grows. If this seems constraining, remember that a single feed can match multiple domains or even a pattern of domains - with some care and forethought, that capability can be used to reduce the number of feeds required in most cases.
Having multiple feeds can impact disk utilization and create extra load on a cluster. When a new feed is defined or restarted, all objects on every volume must be re-evaluated against the feed. This work is in addition to the heath processing, defragmentation, and client activity on each disk, so there can be an impact to these essential activities. Once a new feed is caught up, new writes, updates, or deletes will also incur ongoing feed work for each feed. This work has a smaller impact, mostly focused on CPU in addition to work that is specific to the feed type, such as remote replication or Elasticsearch indexing requests.