Flat vector illustration of a laptop with Outlook showing slow performance, next to an oversized PST folder with 20GB+ warning, speedometer, and split/compact icons

Fix: MS Outlook Runs Slowly or Crashes – Oversized PST File Performance

by

TL;DR: Outlook performance degrades sharply when your PST file exceeds 20GB. The solution is to compact, split, or archive your data. Jump to Solution 3 to learn how to split your PST file safely.

Few things test your patience like a sluggish Outlook—endless “Not Responding” messages, delayed send/receive, and random crashes. If your Outlook has become painfully slow, the most likely culprit is an oversized PST file.

Microsoft sets a recommended limit of 20GB for PST files. When you cross that threshold, performance drops dramatically. At 50GB (the hard limit for Outlook 2013 and later), corruption and crashes become common.

📌 Important – SSD vs. HDD: Your storage drive plays a huge role. A 20GB PST on a modern Solid‑State Drive (SSD) may still perform acceptably. On a traditional Hard Disk Drive (HDD) , that same file can cause severe slowdowns, freezing, and long startup times. If your PST is large and you’re using an HDD, upgrading to an SSD or reducing PST size becomes critical.

What Causes Outlook to Slow Down?

Outlook stores your email, calendar, contacts, and attachments in a Personal Storage Table (PST) file. Over time, this file grows as you accumulate emails. Common causes of performance issues include:

  • PST file size exceeding 20GB – The primary performance threshold.
  • Fragmented PST file – Deleted items leave empty space, causing inefficient access.
  • AutoArchive disabled – Old emails never move out of the main PST.
  • Large attachments stored within the PST – Attachments bloat the file.
  • Corrupted PST file structure – Errors accumulate over time.
  • Add‑ins conflicting – Not directly PST‑related, but can compound slowdowns.

💡 Note on OST Files: If you use an Exchange, Microsoft 365, or Outlook.com account, you may see an .OST (Offline Storage Table) file instead of a PST. The same performance rules apply—keep OST files under 20GB for optimal speed. The steps for compacting, archiving, and splitting are nearly identical.

How to Fix Outlook Slow Performance Due to Oversized PST

Work through these solutions in order. Always back up your PST file before performing major operations.

1. Locate and Check Your PST File Size

First, confirm that a large PST is indeed the issue.

  1. In Outlook, go to File → Account Settings → Account Settings.
  2. Click the Data Files tab.
  3. You’ll see a list of data files. The Name column often includes “Outlook Data File” or your email address. Look at the Size column.
    • If any PST exceeds 20GB, performance will be degraded.
    • At 50GB (or even earlier), corruption is likely.

Note: Outlook 2013 and later use a Unicode PST format with a theoretical limit of 50GB, but performance begins to suffer long before that. Microsoft officially recommends keeping PST files under 20GB for optimal performance.

2. Compact the PST File

When you delete items, the PST file does not automatically shrink—it retains empty space. Compacting reclaims that space.

Before compacting: Empty your Deleted Items folder and manually remove large emails with attachments.

  1. Close Outlook.
  2. Open Control Panel → Mail (Microsoft Outlook) → Data Files.
  3. Select your PST file and click Settings.
  4. Click Compact Now. This process may take several minutes to hours depending on file size.
  5. After completion, reopen Outlook and check performance.

⚠️ The “Delete + Compact” Trap: Simply deleting 10GB of email does not shrink the PST file on your disk until you run Compact Now. Many users delete large amounts of data, see no change in disk space, and assume the deletion failed. Always follow deletions with a compact operation to reclaim space.

3. Split the PST File into Smaller Parts

If your PST is well over 20GB, splitting it into multiple smaller PSTs is the most effective solution.

Method A – Create a New PST and Move Items:

  1. Go to File → New Items → More Items → Outlook Data File.
  2. Give it a name (e.g., “Archive 2024”) and choose a save location.
  3. In the navigation pane, right‑click on folders you want to move (e.g., Sent Items from 2023) and select Move → Copy to Folder → choose the new PST.
  4. Repeat for large folders (Inbox subfolders, large Sent Items, etc.).

Method B – Use Archive Feature (Manual):

  1. Select the folders you want to archive.
  2. Go to File → Tools → Clean Up Old Items.
  3. Set a date to archive items older than, say, 2 years.
  4. Choose an archive file (or create a new PST) and run the archive.

⚠️ Important: After moving items, the original PST may not shrink automatically. Run Compact Now (from step 2) to reclaim space.

4. Enable AutoArchive to Prevent Future Bloat

AutoArchive automatically moves old items to a separate PST file, keeping your main PST lean.

  1. Go to File → Options → Advanced.
  2. Under AutoArchive, click AutoArchive Settings.
  3. Check Run AutoArchive every and set a frequency (e.g., 14 days).
  4. Choose Move old items to and select a destination PST file (or create one).
  5. Set folder‑specific policies by right‑clicking folders → Properties → AutoArchive.

5. Remove Large Attachments

Attachments are the biggest space hogs. You can save attachments to your hard drive and remove them from emails.

  • Use the Advanced Find feature:
    Search → Advanced Find → Advanced tab → Field → All Mail fields → Size → set a condition (e.g., “greater than” 5,000 KB).
  • Delete or save and remove attachments from large emails.

6. Use the SCANPST Tool to Repair Corruption

If Outlook is already crashing, the PST file may be corrupted. Microsoft includes the Inbox Repair Tool (SCANPST.EXE) to fix errors.

  1. Close Outlook.
  2. Search your computer for SCANPST.EXE (typically in C:\Program Files\Microsoft Office\root\OfficeXX\).
  3. Run it, browse to your PST file, and click Start.
  4. If errors are found, check Make backup before repairing and click Repair.

⏳ Important – SCANPST May Appear Frozen: For very large PST files, SCANPST can take a long time and may look like it’s “Not Responding” or frozen. Do not cancel it. Let it run—it could take 30 minutes or more. Canceling mid‑process can corrupt the file further.

7. Create a New Outlook Profile

Sometimes the profile itself becomes corrupted. A fresh profile can improve performance even after you’ve shrunk the PST.

  1. Close Outlook.
  2. Go to Control Panel → Mail (Microsoft Outlook) → Show Profiles.
  3. Click Add, create a new profile, and add your email account.
  4. Under When starting Microsoft Outlook, select Prompt for a profile.
  5. Open Outlook with the new profile and test performance.

8. Use the Microsoft Support and Recovery Assistant (SaRA)

SaRA can automatically diagnose and fix PST‑related issues.

  1. Download the Microsoft Support and Recovery Assistant.
  2. Run the tool and select Outlook → Outlook is slow or freezes.
  3. Follow the prompts. The tool can help with PST size, corruption, and add‑ins.

Related Reading from Mukunda Software Updates

If you’re troubleshooting other Outlook issues, these guides from our blog may help:

Fix: “A Required Authentication Broker is Missing” in Outlook & Microsoft 365
Fix: MS Outlook Folders Disappeared (Only Inbox Visible)

Share this post:

Share on Pinterest Share on LinkedIn Share on WhatsApp