There is not a particular number where things break down suddenly, but generally relational database performance deteriorates as the number of rows in a table increases. Once that’s order of magnitude 100 millions, performance tends to be an issue. Some options you have there:
Also, AxonIQ provides a dedicated EventStore to address high-performance use cases. Some info here: https://axoniq.io/events/2017/10/axoniq-eventstore-yet-another-database.html
It’s currently at the end of its beta testing period, and we plan to release it for production any day now.