How to Fix Outlook Slow Performance: Large OST/PST Files, Indexing, and Add-ins

Why Outlook Gets Slow

Outlook performance degrades gradually as your mailbox grows. Here’s how to diagnose and fix:

Step 1: Check Your Data File Size

Your OST/PST file location: %LOCALAPPDATA%\Microsoft\Outlook\

dir "%LOCALAPPDATA%\Microsoft\Outlook\*.ost" "%LOCALAPPDATA%\Microsoft\Outlook\*.pst"

Performance Impact by File Size

OST/PST Size	Impact	Recommended Action
< 5 GB	Normal	No action needed
5-10 GB	Slightly slower startup	Monitor, reduce sync window to 6 months
10-25 GB	Noticeably slow, search lag	Reduce sync to 3 months, archive old mail
25-50 GB	Very slow, frequent hangs	Urgently archive, compact, consider new profile
> 50 GB	Crashes, data corruption risk	Immediately archive and compact

Top 7 Fixes (Ranked by Impact)

#	Fix	Impact	Where
1	Reduce sync window to 3 months	⭐⭐⭐⭐⭐	Account Settings > Change
2	Move old mail to archive PST	⭐⭐⭐⭐⭐	File > Cleanup Tools > Archive
3	Disable unnecessary add-ins	⭐⭐⭐⭐	Options > Add-ins
4	Rebuild search index	⭐⭐⭐	Options > Search > Rebuild
5	Compact OST/PST file	⭐⭐⭐	Data Files > Settings > Compact
6	Enable Cached Exchange Mode	⭐⭐⭐	Account Settings > Change
7	Create new Outlook profile	⭐⭐	Control Panel > Mail > Profiles

Folder Item Limits

Folder	Recommended Max	Why
Inbox	5,000 items	Outlook renders all items on load
Sent Items	5,000 items	Same rendering overhead
Calendar	5,000 items	Recurring events multiply quickly
Contacts	10,000 items	Autocomplete slows beyond this
Deleted Items	Empty regularly	Counts toward OST size

Add-in Troubleshooting

Symptom	Likely Culprit	Fix
Outlook takes 30+ seconds to open	Add-in loading at startup	Disable all, re-enable one by one
Outlook crashes on Send	Antivirus scanning outbound mail	Exclude Outlook from AV scanning
Search returns no results	Corrupt Windows Search index	Rebuild index from Indexing Options
”Not Responding” when switching folders	Large folder + Online Mode	Switch to Cached Exchange Mode

Troubleshooting

Problem	Solution
OST won’t compact	Close Outlook, rename `.ost`, reopen (it recreates from server)
PST too large to open	Use `scanpst.exe` (Inbox Repair Tool) from Office install folder
Outlook freezes on startup	Start in safe mode: `outlook.exe /safe`
Calendar is extremely slow	Reduce recurring event range; archive old years

Summary

#1 fix: Reduce sync window to 3 months — instant improvement.
Keep OST/PST under 10 GB and folders under 5,000 items.
Disable add-ins you don’t actively use.
Use Cached Exchange Mode, not Online Mode.

Frequently Asked Questions

Why is Outlook so slow?

The most common causes are: an oversized OST/PST file (over 10 GB), too many items in a single folder (Inbox with 50,000+ emails), a corrupt search index, problematic add-ins loading at startup, or Outlook running in Online Mode instead of Cached Exchange Mode. Fixing these can make Outlook feel like new.

What is the difference between OST and PST files?

An OST file is a local cache of your Exchange/Microsoft 365 mailbox — it syncs with the server. A PST file is a standalone data file (archives, exports). Both can grow very large and cause performance issues, but OST problems are fixed by reducing the sync window, while PST problems require archiving or compacting.

How large should my OST/PST file be?

Microsoft recommends keeping OST/PST files under 10 GB. Performance degrades noticeably above 10 GB and Outlook may crash above 50 GB. The default maximum is 50 GB. Reduce size by shortening the sync window (e.g., 3 months instead of 12) or archiving old mail.

Should I use Cached Exchange Mode or Online Mode?

Always use Cached Exchange Mode for best performance. It keeps a local copy (OST) so Outlook works even offline and doesn't wait for server responses. Online Mode fetches everything from the server in real-time, which is much slower.