There are lots of potential causes, and so lots of potential "tweaks" to improve performance. We list those below. If you can do only the first one, you'll probably experience significant relief.
Do you have every email you've received in the last few years in your inbox? Is your Sent Items folder just as packed? When you're in a folder, look at the "item count" in the lower left. If it shows more than 8,000 to 10,000 items, you've got too many, especially in the main folders of INBOX, SENT, DRAFTS and TRASH. This is an IMAP protocol limitation where there is a performance hit once you hit 10,000 emails per folder or if you also have over 100 folders and sub folders. If you do need more than a 100 folders, then the solution is request from support@practicalhost.com to upgrade your mailbox to Exchange 2016 100GB. Exchange is more tolerant to folder count.
This is an IMAP limitation and not an EnsureMail or Outlook limitation.
Create subfolders and move the archival stuff out, particularly out of your Inbox and Calendar. You'll still have access to them, but Outlook won't have a gigantic indexing task every time you do something.
It's important that the subfolders be directly under (formally, "children" of) the root of your mailbox -- marked "Mailbox - [your name]." Creating subfolders under the Inbox, for example, won't really fix the problem.
#2: Did we mention trimming your mailbox down?
It's painful to re-organize all that mail. But it'll make a difference, which is why we mention it again. In general, email software is not intended for use as an electronic document archive.
Use of indexing within Outlook, especially if you're running it on an older Windows and have installed Microsoft Windows Desktop Search, can really slow things down.
In Outlook go to Tools > Options > Preferences tab > Search options. Now uncheck all folders.
This is another process that can slow things down. Go to Tools > Options > Other > Advanced options, then uncheck "Sync RSS Feeds to the Common Feed List" and "Any RSS Feed that is updated appears as a new item". Click OK.
Close Outlook if you have it opened. Right click on the Outlook shortcut and select Run as Administrator. Once you’re logged in, go to Tools > Trust Center. Select Add-ins on the left side of the window, then click on the Go button on the bottom. Uncheck everything except Windows Search Email Indexer.
PST is the file format that Outlook uses. To compact such files , you need to close Outlook first, then go to Control Panel > Mail > Click Data Files. Then double-click "Personal Folders" and choose "Compact now". Then start Outlook.
If you have any software that does indexing and searching of you mailbox, like Google Desktop, consider turning it off too. Or at least turn off the ability to search the mailbox.
#10: Did we mention trimming your mailbox down?