Currently, adding new topics requires restarting mirror maker with the configuration update. So sometimes after doing that it's observed that it takes a long time for data to start moving from source to destination for the new topics.
Wondering why this might be happening and what are some possible options to avoid the same?
If you're using the same config, rather than starting new ones (with a different group.id) as I answered before, then, yes, the consumer group will need to rebalance and adding more and more topics into one consumer group without adding more consumer instances will be slower than less total topics.