The buckets on the slow indexer will have a guid specific to that indexer in their filenames. That data will not replicate to the new indexer unless you increase the replication factor after adding the indexer. So you could do that, wait for everything to replicate, then shutdown the slow indexer and reduce your replication factor back to 2. You will need to be in normal operations mode for that to work, not maintenance mode.
IF for some reason that doesn't work, then the other option would be to clone the slow indexer to the new indexer, shut down the slow indexer, and re-ip the new indexer to the old ip of the slow indexer. This is probably the cleanest method IMHO.
Still I wonder why your slow indexer is slow... Does it have less cpu/memory or slow disk? Or is it a configuration issue. If it's a configuration issue the 2nd method above will not work.
