Designing monthly postpaid invoice module

We currently have an invoice system that generates a monthly invoice by reading all the transactions in the database of a certain user for that month at the billing point of the month.

The invoice module and the transaction module lives in the same monolith and database.

Problem is this process is very expensive and takes a bit of time specially if that user have lots of transactions(millions) for that month.

We were wondering if there is a design pattern /architectural approach for this problem?

We are open to extracting out the invoice module potentially to it's own service, but just trying to figure out how to implement it properly.

Any help is appreciated.