Yeah, that's rough but textbook cold start problem. Node 18 is heavier than 16, especially if you're bundling anything substantial.
Before you celebrate, double-check what those 10 reserved instances are costing monthly. Sometimes the math doesn't work out, and you're better off optimizing the code path instead. Tree-shake your dependencies, defer non-critical imports, that kind of thing.
Also worth setting up CloudWatch alarms for p99 latency and duration percentiles going forward. Catch drift before it becomes a 3am incident next time.