Serverless Cold Start vs Warm Start

Understanding the performance differences between cold and warm starts in serverless architecture

What Users Actually Experience

Cold Start Experience

User's Browser
myapp.vercel.app
Ready to load...

Warm Start Experience

User's Browser
myapp.vercel.app
Ready to load...

Technical Details: What Happens Behind the Scenes

Cold Start

First request or after idle timeout

Progress0%
0ms950ms

Execution Steps:

Container Init
300ms

Starting new container instance

Runtime Load
200ms

Loading runtime environment

Code Load
150ms

Loading function code

Dependencies
250ms

Installing dependencies

Execution
50ms

Running your function

Resource Usage

CPU:High (80-100%)
Memory:High (200MB+)
Total Time:950ms
Warm Start

Subsequent requests with active container

Progress0%
0ms50ms

Execution Steps:

Execution
50ms

Running your function

Container already initialized
Runtime and dependencies loaded

Code already in memory
Ready for immediate execution

Resource Usage

CPU:Low (10-20%)
Memory:Minimal (50MB)
Total Time:50ms

Key Takeaways

Cold Start Challenges

  • • Higher latency (200-1000ms+)
  • • Resource initialization overhead
  • • Unpredictable user experience
  • • More CPU and memory usage during startup

Warm Start Benefits

  • • Minimal latency (10-50ms)
  • • Immediate function execution
  • • Consistent performance
  • • Efficient resource utilization