Nearly all my searches involve a tag. And often I use the same tag in multiple searches during the day.
It’s a bit inefficient that Obsidian searches through all files looking for #my-tag. After all, tags are relatively static – during a day, 98% of files probably don’t have their tags change.
So if Obsidian could simply remember which files had the #my-tag tag, the search process only have to look in that subset of files. That makes search a lot faster! 
My motivation for creating this feature request is the following. Lately I’ve been frequently searching with a specific tag. When I type that tag into the search results, it takes 11 seconds (I timed) before the first results show.
However, that tag is only used in 1.7% of my notes. So several times each day I wait 11 seconds when Obsidian searches in vain through 98.3% of my notes. 
Now 11 seconds isn’t a huge amount of time, but my Vault isn’t even 6 months old. So in a year from now, this problem is even bigger and might take 25-30 seconds.
(I searched the forum for similar requests but didn’t find any.)