How to increase Outlook performance?

Outlook 2016 takes half a minute to open, freezes when switching folders and search crawls. The machine itself is fine, an SSD with 16GB of RAM.

Mailbox is large, ten years of mail. Where do the real performance gains hide?

On a healthy SSD machine, Outlook slowness with a big mailbox comes from four places. In order of payoff:

1

Shrink the local cache. File, Account Settings, double click the account and pull the Mail to keep offline slider to 12 months or less. This is the single biggest win with ten years of mail, the OST drops from tens of GB to a few and everything that reads it speeds up. Older mail stays a click away via the View more on server link at the bottom of each folder.

2

Audit add-ins. File, Options, Add-ins shows the list, and Outlook logs each one's boot cost under File, Slow and Disabled COM Add-ins. Anything adding over a second that you do not actively use, disable. Old PDF and meeting plugins are the usual sinners.

3

Compact the data files. After the slider change the OST holds free space it never returns on its own. Account Settings, Data Files, Settings, Advanced, Outlook Data File Settings, Compact Now. Do the same for any attached archive PSTs.

4

Trim folder item counts. Folders beyond roughly 100,000 items make Outlook grind on every view switch. Archive the worst offenders by year into a separate PST so no single folder carries the decade.

If search specifically stays slow after all four, rebuild the Windows index: Control Panel, Indexing Options, Advanced, Rebuild, then leave the machine on overnight since Outlook search rides on that index. And after everything, measure honestly by cold starting the machine, the difference from step one alone usually turns the half minute into single digit seconds.

Slider to 12 months plus killing two ancient add-ins took startup from 32 seconds to about 5. Compacted and archived the 190k item inbox over the weekend. Feels like a new install.