One quick and dirty way to get faster search is using the settings search prefer recent posts and search recent posts size. That works very well by prioritizing recent posts first to fill search results.
Other than that you would need to do some analysis of the database performance. Check plans, costs, etc. With larger forums you will need more RAM to keep up performance (and tweak PostgreSQL settings accordingly).
If everything else fails you can also try Add Algolia search to your Discourse