Why is Thunderbird so Slow? All Causes, Fixes & Verified Solutions

Written By

Nick Rogers

Published On

January 13th, 2026

Reading Time

9 Minutes Reading

Thunderbird is a widely used open-source desktop email client. It is compatible with Windows, Mac, and Linux devices.

However, many legacy and business users often wonder, “Why is Thunderbird Very Slow?” This can be due to large mailboxes, excessive and unused add-ons, corrupted .msf files, and many more reasons.

This article explains the causes why Thunderbird is getting slow and how to fix that.

Understanding How Thunderbird Email Client Works

Understanding how Thunderbird works helps you dive deep into the causes of “Why Thunderbird is Slow?” and choose the right method to speed it up.

• Mozilla Thunderbird does not store emails inside the application but in a separate profile folder inside your PC.
• Each email folder, like Inbox, Sent, Draft, or any Other custom folder, is stored in a separate folder inside Thunderbird.
• It uses IMAP to sync emails from an email account. A weak connection can also slow down Thunderbird’s speed and functionality.
• Deleting a message inside Thunderbird will not delete it from the profile folder, causing the emails to pile up over time.
• Thunderbird has an .msf file (Mail Summary File) to index emails. Although it does not store emails, it stores the location where a particular email is stored.

This setup makes Thunderbird very slow and causes user issues.

Reasons: Why is Thunderbird so Slow?

Thunderbird, the desktop email client, is often slow to open, load messages, & send or receive emails. This interrupts user productivity and professional email handling. Here are a few reasons behind it.

• Multiple Large Mailboxes (more than 4GB) in one profile
• Piling old emails in the local Thunderbird profile
• Corrupted MSF file of Thunderbird
• Legacy OS or the Thunderbird version
• IMAP Server fault or overload
• Installing a large number of unused Add-Ons
• Antivirus interruption

Affects & Common Reasons: Thunderbird Very Slow Issue

These reasons can cause multiple issues for the user listed below:

Thunderbird getting slow due to any of these reasons makes it difficult for the user to send or receive emails and decreases daily productivity.

Thunderbird Very Slow? Proven Fixes to Improve Speed & Performance

The following fixes are verified and tested by experts at 4n6 to improve speed and performance by tenfold if your Thunderbird is so slow, loads messages, opens, or freezes unexpectedly.

General Fixes if Thunderbird Slow on Windows & Mac

Many basic troubleshooting steps, like updating the Operating System, Thunderbird Application, or disabling the antivirus, can help resolve Thunderbird getting slow issues.

Fix Thunderbird’s slow performance on Windows 11 & Other versions by disabling antivirus. Antivirus interferes with incoming and outgoing messages as well as conflicts with indexing, making the application slow to load mailboxes.

Fix Thunderbird Slow After the Update

Thunderbird, being an open-source email client, often releases new updates and versions.
Performance and speed issues after the update are due to conflict of Add-Ons and the new application updates.

Disable or uninstall add-ons to fix Thunderbird freezing issues.

Steps to Disable or Remove Add-Ons to Fix Thunderbird So Slow Issue

1. Open Thunderbird.
2. Click on the Hamburger menu on the top right (Display Thunderbird Menu).
3. Click on Add-Ons and Themes.
4. Choose Extension from the left panel.
5. Manage extension >> Disable.
6. You can click on three dots >> remove to uninstall the extension.

How to Fix Thunderbird Slow with Large Mailboxes

Large mailboxes, often greater than 4GB or with 10000+ messages in a folder, can cause Thunderbird to slow down.

You can try our multiple fixes, like deleting trash, junk, or compacting the profile folder, to resolve the Thunderbird is very slow issue.

Fix 1: Clear out Trash and Junk

The Junk and Trash folder unnecessarily increases the mailbox size and slows down Thunderbird performance.

1. Open Thunderbird
2. Go to the Mail.
3. Go to the Trash Folder >> Right-Click.
4. Choose the Empty Deleted option.

Like how you cleaned out Trash to speed up Thunderbird, go to Spam >> Right click and choose Empty Spam to clear the spam.

Fix 2: Compact Thunderbird Folder

Compact is a built-in option offered by Thunderbird to wipe out already deleted emails in the profile folder. This, in turn, helps the user to speed up Thunderbird. It is fairly simple.

1. Open Thunderbird.
2. Right-click a large email folder.
3. Click Compact.

Thunderbird also shows how much space is saved after compacting, in turn solving the slow issue due to large mailboxes.

Fix Thunderbird Slow to Load Messages Issues

It is a common user issue: “Why does Thunderbird take so long to open or load messages?”

A corrupted or very large MSF file(Mail Summary File) is the primary reason behind it.

Fix 1: Recreate Thunderbird MSF

The MSF file is the index of all emails stored inside Thunderbird. If it is corrupted, Mailboxes and messages will not be opened in Thunderbird, or will be too slow to open.

Recreating it is easy; here are the steps you need:

1. Close the Thunderbird Application
2. Go to C:\Users\\AppData\Roaming location on your File Folder.
3. Open the Thunderbird >> Profile Folder.
4. Go to IMAP Mail >> delete the MSF file.
5. Now launch Thunderbird, and your correct MSF file will be regenerated.

In turn, the performance of Thunderbird will no longer be slow to open mailboxes.

Fix 2: Turn OFF Global Indexer

Global search and indexer is a native option inside Thunderbird. It lets you search emails, folders, etc., under one section in Thunderbird.

To turn off the Global search and indexer to fix the Thunderbird very slow issue-

1. Open Thunderbird.
2. Go to Settings >> General.
3. Go to Find in Settings >> search Global Indexer.
4. Uncheck Enable Global Search & Indexer.
5. Restart Thunderbird.

How to Fix Thunderbird Very Slow with Gmail?

Gmail is a popular email service widely used along with Thunderbird. Many users complain that Thunderbird takes time to load Gmail mailboxes.

This is because Gmail does not have a standard folder structure. Instead, it has labels. Along with this, all the emails are stored in a duplicate sense in the All Mail folder. This unnecessarily increases the mailbox size, causing Thunderbird to slow down. To resolve this, simply empty the All Mail folder.

When Manual Methods Fail to Speed Up Thunderbird?

Manual in-built approaches in Thunderbird, like updating the application, disabling antivirus, compacting the folder, and deleting all mail with Gmail, can help speed up Thunderbird performance.

However, it is limited to an extent. When the issue is related to server-side sync, quota limits, or profile-level corruption, Thunderbird natively can’t fix that.

Here are a few situations when you will need professional help to fix Thunderbird so slow:

• When folder data is corrupted, you need proper restoration and recovery.
• You have bulk-organization-level data associated with the Thunderbird profile.
• In the case you have a legacy profile with an older configuration, you need a proper restore.

Professional Solution to Fix Thunderbird so Slow Issues

4n6 MBOX Converter Tool is an expert trusted software to securely restore corrupted data in bulk. It can help you rebuild your profile with all the data, including emails, attachments, metadata, and folder structure, with ease. A professional way to fix corrupted or older configuration folders, which can speed up Thunderbird performance. It is a reliable solution where the manual method can be too risky or fail to resolve Thunderbird’s slow issue. Download the tool today.

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How to Fix Thunderbird so Slow using the Tool?

The 4n6 Tool helps you speed up Thunderbird performance by re-configuring files, making the data clean and healthy. This makes Thunderbird faster and easier to load.

Steps to use the 4n6 advanced tool to fix the Thunderbird keeps freezing issue:

1. Download and install the advanced tool on your device.
2. Click on the Open icon on the top-left.
   
3. Click on Desktop Email Client and Choose Thunderbird.
4. Choose configured Thunderbird accounts.
   
5. Preview all the Thunderbird data.
6. Click on Export and choose Thunderbird from the saving option.
   
7. Click Save on the top-right.

Now restart Thunderbird, and the 4n6 tool has resolved the “Thunderbird so slow” issue.

Why Choose the Tool to Fix Thunderbird Extremely Slow Issue?

The 4n6 advanced tool helps you speed up Thunderbird performance by reconfiguring the profile error-free. Here are the powerful features of the tool that make it unique and reliable:

• Restore corrupted data with ease to fix the Thunderbird keeps freezing issue.
• Works 100% offline and follows data privacy guidelines like HIPAA, CAPA, and GRDP.
• Auto-fetches and re-imports email data into the Thunderbird profile.
• Preserves header, body, footer, metadata, formatting, attachment, and folder structure.
• Advanced algorithms to handle large data in a faster way, which reduced the downtime.

Expert Advice

Users have come up to 4n6 experts with issues like “Why is Thunderbird so slow?” There can be many reasons, like large mailboxes, an antivirus interference, an older OS version, legacy profile structure, or corrupted data, etc., that can slow down or freeze Thunderbird.

We have noticed that, in most cases, general troubleshooting steps and native methods like updating Thunderbird and compacting the profile can be a reliable option.

However, if the issue is related to profile corruption and you are dealing with important data in bulk, using a professional tool becomes a necessity.

Tip: To avoid data loss in any scenario, back up the Thunderbird profile using  4n6 Thunderbird Converter.