Outlook is one of those programs that feels fine for years — and then, one day, takes thirty seconds to open an email. Or freezes when you try to send. Or shows the dreaded "Outlook is starting" for far too long.
The good news is there's almost always a real reason, and most of them are fixable. Here are the things we check most often in our Tenterden workshop when someone brings in a slow Outlook.
1. A huge mailbox
This is by far the most common cause. Outlook stores email in a local file (called an OST or PST), and once that file gets above a certain size — often around 10–20GB — performance starts to suffer. Above 50GB it's usually painful, and above 100GB Outlook can become almost unusable.
What to do: Have a clear-out. Empty the Deleted Items folder (which often contains years of forgotten email). Check the Sent Items folder, which can be even bigger. If you have huge attachments, consider archiving older mail to a separate location.
2. A damaged or bloated Outlook profile
Outlook profiles can become corrupted over time — especially after Windows updates, account changes or sync interruptions. The symptoms are usually slow startup, freezes when switching folders, or unreliable syncing.
What to do: Creating a fresh Outlook profile is usually the quickest fix here. It sounds drastic but it's actually one of the safer Outlook procedures — your email lives on the server, not in the profile, so a fresh profile re-downloads everything cleanly. We do this regularly and can talk you through what to expect.
3. Too many add-ins
Every add-in that loads into Outlook costs you a little bit of startup time and a little bit of responsiveness during use. PDF tools, CRM integrations, scanner software, antivirus email scanners — they all add up.
What to do: Outlook has a built-in list of add-ins (File → Options → Add-ins) and you can disable the ones you don't need. Be conservative — disable, restart, see if things improve, repeat. Some add-ins are useful; some are quietly slowing everything down.
4. A misbehaving OST file
The OST is the local cache of your mailbox. If it becomes corrupted or fragmented, Outlook can become slow, freeze, or show sync errors. Sometimes Windows itself reports that the OST file has "exceeded the limit."
What to do: Outlook can rebuild the OST from scratch — and again, since the real data is on the server, this is a safer operation than it sounds. The new OST downloads fresh from the mailbox and the old one is removed.
5. Antivirus software scanning every email
Most modern antivirus products no longer need to scan email separately — Windows Defender, Office 365 and your email provider already do this on the server side. But some older or third-party security suites still hook into Outlook and slow it down meaningfully.
What to do: Check whether your antivirus has an "email scanning" option, and consider turning it off. The chances of catching anything that the server didn't already catch are very low, and the speed difference is sometimes substantial.
6. The PC itself is slow
Sometimes Outlook isn't the problem at all — the underlying computer is just struggling. If everything is slow, not just Outlook, the answer might be an SSD upgrade, a memory upgrade, or sorting out a virus or malware infection.
What to do: If everything is slow, start with the computer rather than with Outlook. Our SSD upgrade page covers the single most cost-effective fix for older PCs.
7. Sync problems with the server
If you're using Outlook with Microsoft 365 or another modern email server, Outlook constantly syncs in the background. If that sync is being interrupted — by a flaky network connection, a server-side problem, or an account issue — Outlook can spend its time retrying and feel slow as a result.
What to do: Check the Outlook status bar (bottom-right). If it says "Trying to connect" or "Disconnected", the issue is likely network or account-related rather than Outlook itself.
When to ask for help
If you've worked through the obvious things — emptied Deleted Items, disabled odd-looking add-ins, checked the connection status — and Outlook is still painful, it's usually time to ask someone to take a proper look. Some Outlook fixes are easy on the surface but have side effects if done wrong, and a corrupted profile or OST is the kind of thing that benefits from a calm, careful approach rather than guessing.
We sort Outlook problems regularly — for home users on Microsoft 365, for small businesses with shared mailboxes, and for people who just want their email to behave again. See our Outlook & email support page for more, or give us a ring on 01580 764404.