There isn't an easy answer.
If you run the indexer, it will gobble up CPU resources - lots at first while it indexes everything, and then somewhat less once that's done and it's just updating.
100k mails will give it a big job if it is to index them all. I could see it taking weeks to do that lot, and then a fair old time to search the index each time you ask it. I archive off old mail, by subject etc, and don't index the archive. I don't even notice the indexer overhead.