---
title: Bytecode profiler walkthrough
description: From a switch to the expression that is slow: start a session, use the pages, stop it, read the files and the widest expressions, open your source as a heatmap, jump from a Hot spots row, ask Lensy, share it, and let a rule start a session by itself. A narrated video with captions.
icon: lucide:play-circle
---

# Bytecode profiler walkthrough

From a switch to the expression that is slow: start a session, use the pages, stop it, read the files and the widest expressions, open your source as a heatmap, jump from a Hot spots row, ask Lensy, share it, and let a rule start a session by itself. BoxLang+.

<video class="lensy-trailer" controls playsinline preload="metadata" poster="../../assets/videos/walkthrough-profiler.jpg" aria-label="The Bytecode profiler walkthrough"><source src="../../assets/videos/walkthrough-profiler.mp4" type="video/mp4"><source src="../../assets/videos/walkthrough-profiler.webm" type="video/webm"><track kind="captions" src="../../assets/videos/walkthrough-profiler.vtt" srclang="en" label="English" default></video>

## What it shows

1. **Before you start**: Two things must be on.
2. **Start a session**: It records the whole server.
3. **Stop and save**: A session always ends.
4. **Open a session**: Files, with a badge.
5. **The widest expressions**: What ran, how often, how long.
6. **The heatmap**: Your own source, line by line.
7. **Read the numbers**: Lines first, then segments.
8. **Close the file**: Other badges say why there is no heat.
9. **From Hot spots**: Go from a slow row to its expressions.
10. **Ask Lensy**: It reads the session and says what to change.
11. **Share it**: Your people, your channels, or any address.
12. **Rules**: A request that starts a session.
13. **The rule fires**: The first request is the first one recorded.
14. **Settings**: Everything is a setting.

## Read more

[Bytecode profiler](../console/bytecode-profiler.md) and [Find and fix a slow page](../guides/find-slow-code.md).

See all the [walkthroughs](../walkthroughs.md).