AWS Lambda cold starts: causes and fixes

AWS Lambda cold starts occur when no warm container is available, requiring image download, runtime startup, and init code execution. Cold start latency varies by runtime—Go is fastest (50-200ms) while Java is the worst (1-4s). The most effective fixes, ranked by ROI, are slimming container images, moving heavy initialization outside the handler, enabling Java SnapStart, and using provisioned concurrency as a last resort.