Hi @techcoor
If you want to convert your POP account to an IMAP account, you'll need to remove the current POP3 account (this will not delete any data) and then add the IMAP account.
After you have added your IMAP account and if your mailbox space permits it, you could optionally transfer your current emails, which you downloaded via POP3, and folder structure to your IMAP account. Here we could simpy import the PST file to the newly created IMAP account (this operation can make all your messages and folder structure shown in your Outlook). The previous .pst location can be found in outlook > Files > Account Settings > Account Settings > Data Files > Double click the account below you will find and copy the complete file path.
Import email, contacts, and calendar from an Outlook .pst file
You could also take a reference at the official document here which introduces about the difference between POP and IMAP
And this will be helpful to you as well: How to troubleshoot performance issues in Outlook
If you have a large .pst or .ost file, you may experience application pauses while you perform typical operations in Outlook. These typical operations include reading, moving, and deleting email messages.
When you use Outlook, the following .ost size guidelines generally apply: 
If you use .ost or .pst files
If you have lots of items in any single folder, you may experience performance issues during certain operations in Outlook while you use local data files. If you use Outlook to connect to a mailbox that is located on a Microsoft Exchange Server and use Cached Exchange Mode, Outlook synchronizes email and other items to an Offline Outlook Data (.ost) file. If you use Outlook to connect to other email accounts, such as POP3 or IMAP, Outlook stores email and other items to a local Outlook Data (.pst) file. These performance issues are especially noticeable when you switch into and out of folders that contain many items.
If you have more than 100,000 items in a single folder in Outlook 2010 and later versions, views other than Arrange By: Date can be slower. We recommend that you move several items in these larger folders to separate folders in the same store or to an archive store and that you use Arrange By: Date when you run Outlook. For more information, click the following article number to view the article in the Microsoft Knowledge Base:
2768656 Outlook performance issues when there are too many items or folders in a Cached mode .ost or .pst file folder
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