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Outlook fix - classic

Outlook running slow or hogging memory

If classic Outlook is slow to open, laggy or eating memory, the usual culprits are heavy startup add-ins and a bloated OST/PST data file. Disable non-essential COM add-ins, keep the mailbox trimmed, empty bulky folders and repair Office - or we'll speed it up for you remotely in minutes.

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By the 365 Techies team · Reviewed 6 October 2026

The quick answer

Two things slow Outlook down most

A slow, laggy Outlook often comes down to two things: a heavy add-in loading at startup, or a giant data file (OST/PST) that Outlook has to wade through. Tackle these first on classic Outlook for Windows:

  1. Disable startup COM add-ins. Add-ins from PDF tools, CRMs, antivirus and old toolbars can each add seconds to launch. Go to File > Options > Add-ins, at the bottom set Manage: COM Add-ins and click Go, then untick anything you don't actively use. Restart Outlook and see how it feels.
  2. Shrink the mailbox Outlook keeps offline. On an Exchange or Microsoft 365 account, go to File > Account Settings > Account Settings > Change and drag the Download email for the past slider (called Mail to keep offline in older versions) down to 3, 6 or 12 months instead of All. This keeps the local OST small and fast. Older mail still lives on the server and stays searchable online.

A quick test to confirm add-ins are the problem: press Windows key + R, type outlook.exe /safe and press Enter. If Outlook is nippy in Safe Mode (add-ins off), you've found your culprit.

// WINDOWS PCS ONLY

We do not support Outlook on Apple Macs, iPads or iPhones. Apple does not allow our remote-support tool to take control of an iPad or iPhone (an Apple restriction), and our remote support covers Windows PCs only. On an Apple device, Microsoft’s own Outlook help is the place to start.

// FIX IT WITH ME · STEP BY STEP

Fix it with me, one step at a time

The steps from this guide, one at a time, in the order that fixes the most cases fastest. Tell it what happened and it shows the next step. Nothing here leaves your device.Last reviewed 6 October 2026

Checking which Windows you have…STEP 1 OF 6
  1. 01Disable startup COM add-ins

    Go to File > Options > Add-ins, set Manage: COM Add-ins, click Go and untick anything you don't use. Restart Outlook.

    Is it working now?

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Deeper fixes

Trim, tune and repair

If it's still sluggish, work through these - they clear the most common causes of high memory and a laggy interface:

  1. Empty the bulky folders. Right-click Deleted Items > Empty Folder and clear out Junk. Thousands of old items in these folders bloat the data file and slow everything down.
  2. Keep the OST/PST size sensible. A data file that's grown to tens of gigabytes is the classic cause of a slow, laggy Outlook - startup, sync and search all suffer. Trimming the offline slider (above) and clearing junk keeps it lean.
  3. Check graphics rendering if it's stuttery. On older builds of classic Outlook there was a File > Options > Advanced > Display tickbox called Disable hardware graphics acceleration. Microsoft has since removed that option from current Microsoft 365 builds - Office now switches problem graphics cards to software rendering automatically, so there's usually nothing to toggle. If scrolling still stutters, update your graphics driver, or in Windows go to Settings > System > Display > Graphics, add Outlook and set its GPU preference.
  4. Repair Office and stay updated. Right-click Start > Installed apps > Microsoft Office / 365 > Modify > Quick Repair. Then run File > Office Account > Update Options > Update Now and keep Windows Update current - performance fixes ship regularly.

Good to know: none of this deletes your emails. Trimming the offline slider only changes how much is cached on this PC; on Exchange, 365 or IMAP everything stays safe on the server and re-syncs. If you're on a POP account (mail stored only locally), tell us first and we'll back it up before any changes.

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// GOOD QUESTIONS

Frequently asked

Why is my Outlook so slow to open?

The two big causes are heavy add-ins loading at startup and a bloated OST/PST data file. Disable non-essential COM add-ins under File > Options > Add-ins, and shrink the offline mail slider to 3-12 months. Test with Safe Mode (outlook.exe /safe) - if it's fast with add-ins off, an add-in is the problem.

How do I stop Outlook using so much memory?

Trim the local data file: empty Deleted Items and Junk, reduce cached mail to 3-12 months so the OST stays small, and disable add-ins you don't use. A giant OST/PST is the usual reason Outlook hogs memory and feels sluggish. An Office repair often helps too.

What is the offline mail slider and is it safe to reduce it?

On Exchange and Microsoft 365 accounts, Outlook keeps a local copy of recent mail (the OST) so you can work offline. The slider under Account Settings > Change sets how far back that copy goes - 1, 3, 6, 12 or 24 months, or All. Reducing it to a few months keeps Outlook fast and doesn't delete anything - older mail stays on the server and is still searchable online.

Where's the 'disable hardware graphics acceleration' option gone?

Microsoft has removed that tickbox from current Microsoft 365 builds of classic Outlook. Office now detects problem graphics card and driver combinations and switches to software rendering on its own, so there's usually nothing to toggle. If Outlook still stutters, update your graphics driver or set Outlook's GPU preference under Windows Settings > System > Display > Graphics.

Does a big PST or OST file really slow Outlook down?

Yes - it's a common cause of a slow, laggy Outlook. Files that grow to tens of gigabytes make startup, syncing and search all crawl. Keeping the file trim (reduce cached months, clear Deleted Items and Junk) is often the most effective fix.

Can you speed up Outlook without me doing all this?

Absolutely. Via Splashtop SOS we phone first, you click one link, and you watch us find the add-in or bloated data file that's dragging Outlook down and fix it - usually in a few minutes. We never need your email password and access ends the moment we're done.

// Still stuck?

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We'll connect securely, spot the add-in or bloated data file that's dragging it down and get Outlook snappy again - usually in a few minutes, while you watch.

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