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If you care about replay time and ability to respond to patches, you will not likely be using MIRA for a witness node. It will allow you to use low-end hardware but the replay time increase is not worth the cost savings unless it is a back up of a backup node.

Good question!

Nah replay uses ram, it will be about the same speed. Unless you downgrade server specs and it will take longer depending how much ram you have. I suspect it wouldn't be a wise choice to downgrade your witnesses specs for the time being until we get some real stats on this.

Here is a rundown: https://github.com/steemit/steem/blob/master/doc/mira.md