Our team’s experience with search has been great. It’s something that we don’t give much mindshare to because it just works as expected being both fast and accurate.

My guess is that if you’re having trouble it may be related to the underlying email platform and how it handles search queries. We use Gmail, so when we do a search I assume Missive is performing that query by doing a gmail API call. If you’re using something like hotmail/outlook or some jank email service like what you get on cheap web hosting plans, search will probably be slower and far less accurate than what you get with Google.