First requests or after idle: high tail latency/p99 (seconds-minutes). Dashboard shows long 'pending' (queueing) or uneven invocation times (first per container slow).

Cold starts occur when no warm container available: queueing delay waiting for boot (~1s Modal base + user code like imports/model loads) + first-invocation init (e.g. model to memory, sequential IO). Subsequent calls reuse warm containers.