1. Introduction

## 1.1 Brief explanation of the tutorial's goal
The goal of this tutorial is to provide best practices for optimizing the performance of Firebase Cloud Functions. We will deep dive into the methods of reducing cold start times, reusing connections, and structuring your code for efficiency.

## 1.2 What the user will learn
By the end of this tutorial, you will understand how to:
- Reduce cold start times in Firebase Cloud Functions
- Reuse connections effectively
- Structure your code for optimal efficiency

## 1.3 Prerequisites
- Basic knowledge of JavaScript
- Basic understanding of Firebase and Firebase Cloud Functions

2. Step-by-Step Guide

## 2.1 Reducing Cold Start Times
Cold starts occur when your function hasn't been used for a while and has to start from scratch. Reducing cold start times can greatly improve the user experience.

### Best practices
- Keep your functions warm: You can keep functions warm by scheduling a timer to ping your function every few minutes.
- Minimize code at the global level: Code outside of the function runs during a cold start, and can slow things down.

## 2.2 Reusing Connections
Creating new connections, such as to a database, can be time-consuming and can slow down your functions.

### Best practices
- Reuse connections: Instead of creating a new connection every time your function runs, create a connection outside of your functions and reuse it.

## 2.3 Structuring Your Code for Efficiency
How your code is structured can have a big impact on your function’s performance.

### Best practices
- Use lazy-loading: Load modules as you need them, instead of loading everything upfront.
- Use Cloud Firestore triggers efficiently: When using Firestore triggers, be aware of how many times your function will be called.

3. Code Examples

## 3.1 Reducing Cold Start Times
```js
// Global code runs during cold start
const admin = require("firebase-admin");
admin.initializeApp();

exports.myFunction = functions.https.onRequest((request, response) => {
// Function code runs every time
// ...
});
```
In the above example, the Firebase Admin SDK is initialized during a cold start. The rest of the code runs every time the function is triggered.

## 3.2 Reusing Connections
```js
// Outside your function
const admin = require("firebase-admin");
admin.initializeApp();

exports.myFunction = functions.https.onRequest((request, response) => {
// Use the already initialized admin
let db = admin.firestore();
// ...
});
```
Here, we initialize the Firebase Admin SDK and create a connection to Firestore outside of our function. This allows us to reuse the connection every time the function is triggered.

4. Summary

In this tutorial, we have covered how to reduce cold start times, reuse connections, and structure your code for optimal efficiency in Firebase Cloud Functions.

5. Practice Exercises

## 5.1 Exercise 1: Reducing Cold Start Times
Try to refactor your existing Firebase Cloud Functions to reduce cold start times. Remember to minimize global code and keep functions warm.

## 5.2 Exercise 2: Reusing Connections
Refactor your Firebase Cloud Functions to reuse connections. Create a connection outside your function and use it within your function.

## 5.3 Exercise 3: Structuring Your Code for Efficiency
Refactor your Firebase Cloud Functions to use lazy-loading and Firestore triggers efficiently.

Pro Tip:

Always test your functions extensively to ensure they still work as expected after refactoring.