Microsoft 365 Running Slowly? Common Problems and How to Fix Them

Microsoft 365 running slowly is one of the most disruptive, and frustratingly common, complaints from SME teams across the UK. When Outlook takes an age to load, Teams calls break up mid-sentence or Word freezes on a document you need right now, productivity does not just dip; it stops. The good news is that most causes are identifiable and fixable, often without a full system overhaul.

What Are the Symptoms of Microsoft 365 Running Slowly?

Slow performance rarely announces itself clearly. Instead, it creeps in as a collection of small irritations that gradually add up to a significant drain on working time.

Common symptoms include Outlook hanging when loading emails or switching between folders, Teams lagging during video calls or taking several seconds to display messages and OneDrive syncing files at a crawl or failing to sync at all. You might notice that opening a Word or Excel file from SharePoint takes noticeably longer than it used to. Browser-based apps such as the Microsoft 365 web portal may time out or respond sluggishly, even on a machine that handles everything else without issue.

If multiple people in your business are reporting the same problems at the same time, that points towards a network, server or tenant-level issue rather than a single device fault. If only one person is affected, the cause is more likely local: their machine, their browser or their individual account settings.

The Top 10 Common Problems with Microsoft 365 Performance

Understanding the root cause matters far more than applying a quick fix that masks it. These are the ten problems we see most frequently when businesses ask us to investigate sluggish Microsoft 365 performance.

1. Outdated Software or Operating System

Microsoft regularly releases updates that include performance patches. Running an older version of Microsoft 365 or an outdated version of Windows means your software is not benefiting from those improvements. Updates fix known bugs, close compatibility gaps and often resolve speed issues that users have been living with for months.

2. Insufficient Hardware Resources

Microsoft 365 is a demanding suite of applications. If a machine is running with too little RAM, an older processor or a nearly full hard drive, the software will struggle to perform. This is particularly common on older devices that predate the move to cloud-based working.

3. Corrupted Outlook Data Files

Outlook stores data locally in a file format known as a PST or OST file. Over time, these files can become large, fragmented or corrupted. A bloated or damaged data file is one of the most frequent causes of Outlook running slowly and it is something many users never think to check.

4. Too Many Add-ins Running

Every add-in loaded into Outlook or other Microsoft 365 apps consumes memory and processing power. Businesses accumulate these over time, often without realising. Disabling unnecessary add-ins is a quick win that can make a noticeable difference to load times.

5. Poor or Inconsistent Internet Connectivity

Microsoft 365 is cloud-first software. A slow, unstable or congested internet connection will always translate into slow application performance. This is particularly relevant for businesses operating from older premises with ageing broadband infrastructure or shared network connections.

6. DNS Configuration Issues

When your devices connect to Microsoft 365 services, they rely on DNS to find the right servers quickly. Misconfigured or slow DNS settings force that lookup process to take longer than it should, adding latency to every single cloud interaction your team makes.

7. Antivirus or Firewall Interference

Security software is essential, but a poorly configured antivirus tool or firewall can actively slow Microsoft 365 by scanning every file sync, every email attachment and every cloud transaction in real time. The fix is not to remove security software; it is to configure it correctly so it works with Microsoft 365 rather than against it.

8. Overloaded OneDrive or SharePoint Sync

Syncing very large libraries, or syncing folders that contain tens of thousands of files, places significant demand on local resources. Teams often set up sync without realising how much it will affect the machine's overall performance, particularly during the initial sync process.

9. Microsoft 365 Tenant or Licensing Configuration

Some performance issues originate at the tenant level: the way your Microsoft 365 environment has been set up and configured by whoever manages it. Suboptimal settings around conditional access policies, email routing or licence assignments can cause slowness that no amount of local troubleshooting will resolve.

10. Background Processes and Windows Startup Apps

Devices that carry a long list of startup applications compete with Microsoft 365 for resources from the moment they boot up. Background processes running updates, syncing data or indexing content all compete for the same RAM and CPU that Microsoft 365 needs to run smoothly.

How to Fix Microsoft 365 Running Slowly: Practical Steps

Some of these fixes are straightforward enough for a confident user to attempt. Others require someone with the right access and technical knowledge to do safely.

Quick Wins You Can Try Today

Start by ensuring Microsoft 365 and Windows are both fully up to date. Then open Outlook, go to File, Options and Add-ins and review what is enabled. Disable anything you do not actively use. Check your internet speed using a free tool such as Speedtest by Ookla and compare the result against what your broadband package promises. If your speed is significantly lower than expected, contact your provider.

Clearing the Outlook cache is another straightforward step. In Outlook, go to File, Account Settings, select your email account and reduce the amount of mail kept offline. This forces Outlook to work with a smaller local data set, which often improves responsiveness immediately.

When to Involve Managed IT Support

Several of the causes listed above require administrator access to your Microsoft 365 tenant, your DNS settings or your network infrastructure. Attempting to reconfigure these without the right knowledge risks creating new problems while trying to solve existing ones. This is where managed IT services become genuinely useful.

A managed IT provider can audit your entire Microsoft 365 environment, identify whether the slowness originates at the device, network or tenant level and apply targeted fixes without disrupting your team's working day. That proactive approach, catching issues before they become business disruptions, is what separates reactive break-fix support from a properly managed service.

For SMEs in Herefordshire and the surrounding region, our team provides exactly this level of ongoing support. You can read more about how IT managed services work for local businesses or explore whether it is time to change your IT provider if your current support has not been addressing these kinds of performance problems.

For businesses that have not yet made the move to a fully managed model, our blog on why you need managed IT services sets out the commercial case clearly.

Frequently Asked Questions

Why is Microsoft 365 suddenly slow when it was fine before?

A sudden change in performance usually points to a specific trigger: a recent update, a change to your network, a newly installed application or a configuration change in your Microsoft 365 tenant. Check whether any of these happened around the time the slowness started. If you cannot identify the cause, a managed IT provider can run a diagnostic to trace it.

Does Microsoft 365 slow down on older computers?

Yes. Microsoft 365 has increased its hardware demands as the suite has grown. Devices that were adequate three or four years ago may now struggle, particularly if they are running multiple apps simultaneously or handling large OneDrive sync libraries. A hardware audit is often the starting point for resolving persistent performance issues on older machines.

Can a slow internet connection cause Microsoft 365 to run slowly?

Absolutely. Microsoft 365 depends on a stable, fast connection to its cloud servers. Slow or inconsistent broadband will affect Teams calls, email loading, SharePoint access and OneDrive sync. If your broadband speed is the bottleneck, no amount of software configuration will fully compensate for it.

How much does it cost to get Microsoft 365 performance issues fixed?

The cost depends entirely on the cause and the scope of work involved. Some issues take minutes to resolve; others require planned work across your network or tenant. Our blog on how much managed IT services cost gives a clear overview of what SMEs typically invest in this kind of support., If Microsoft 365 running slowly is costing your team time and affecting how your business operates, Wyvern Business Systems can help. We work with SMEs across Herefordshire and beyond to diagnose and resolve Microsoft 365 performance issues as part of a fully managed IT service. Contact our team today to arrange a no-obligation conversation or visit our managed IT services page to find out how we support businesses like yours every day.