java – Creating an AD module for the application

I have application A, where I would like to add features for advertising / collaboration.

The module must have its own database, the application A must communicate with its API to extract information from the database of this module.

The database should contain information on companies and offers, etc. However, I could do it without this module and put everything in the application A.

I am new to microservices, so I ask the most important question: would not communication be slow? Is it worth it to do like this?

How should I decide if creating a microservice is worth it?

And if I decide to do it that way, what is the fastest way to communicate? I'd rather go against communication via HTTP (for example, stay api), but I'm not sure which email service I should use.

Thanks for the help.