This is a potential downside of this architecture. As others have already said, there are mitigations, but fundamentally each edge request is going to be more costly (time or money) to serve than having it served by a single machine/DB.
One of our engineering principals is to avoid premature optimisation, which is possibly one of the reasons our architecture has grown in this way. So far, whenever we've needed to fix a performance issue we've been able to solve it locally rather than change the architecture.
At the business level, we've been optimising for growth rather than costs, but this could change in the future, at which point we may need to reconsider our architecture. But for now it's working for us.
One of our engineering principals is to avoid premature optimisation, which is possibly one of the reasons our architecture has grown in this way. So far, whenever we've needed to fix a performance issue we've been able to solve it locally rather than change the architecture.
At the business level, we've been optimising for growth rather than costs, but this could change in the future, at which point we may need to reconsider our architecture. But for now it's working for us.