I encrypted healthcare data at a previous job. It was indeed making the app much less efficient. Each health institute had their private keys. I considered to have keypairs per user, but it would have been a bit too much work but more neat.

The indexes were a challenge. I was using some argon2d key derivation algorithm to hash the values to use in some indexes. For example if you want to get all the records with the value "toto" in a field, you derive toto (with some common salt), and then you can look in the index all the documents that have the same derived value.

It did leaks some information and some values couldn't be indexed like this because that would leak too much. So sometimes, we had to fetch all the documents from the database and filter on the application level. We also sometimes didn't encrypt the datetimes so we could do efficient queries on specific time ranges.

To be honest, I did that mostly for fun. I know that some people are content with the managed encryption from their favourite cloud provider.

Health data primarily.

You aim to encrypt the fields you can, without hampering usability too much.

Anything you need to be able to search for (name, ssn) to find patients, or filter on for reports, is generally plaintext.

More sensitive things such as "that patient has aids" you'll have to decide if you want to encrypt it, or do a massive select from the DB anytime you need statistics on it. (Or better yet, encrypt it, but store an anonymized tracker elsewhere. But this is less useful for cureable diseases)

It depends on use case but for example in EU a lot of PII needs to be encrypted for the medical/pharmaceutical Services. Of course health data needs to be encrypted as well as financial data etc.

* API keys

* Health data

* Financial data