What's the User SPI Communication Protocol Recommendations

We have an user service that has its own database and is responsible for user datas.

We are writing a user spi to federate these user to keycloak.

At first glance, it seems like the easiest way to communicate with this service is by using HTTP. However, due to HTTP overhead, we’d like to try other possible solutions (gRPC, NATS, etc…)

I’m basically asking the community what are the communication protocol they are using considering there is a service responsible for holding user data and why they made this choice. (we wont use direct database access)

Thanks in advance,

Romain.