Most products don't need to be built for millions of users on day one. But a few early choices make scaling much easier later.
Keep the data model clean
A clear, well-indexed data model is the single biggest factor in long-term performance.
Separate what changes at different speeds
Keep the public site, the API and background jobs independent so each can scale and deploy on its own.
Measure before optimising
Add basic performance monitoring early. You can't fix what you can't see.
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