Office installed in iMac

First ensure you have the most up-to-date version of Office for Mac installed on your system. If you do, then please restart in Safe Mode (this will clear some caches) and restart normally and then re-test Office. If the problem persists, please reach to Microsoft for support on their product.

Safe Mode Reset link

If your machine has a HDD, it's my experience this is the cause. I put up with that for years with a 2015 21.5" iMac and finally replaced the HDD with an SSD (thanks iFixit) and now that problem no longer exists. My wife's 2015 27" had a fusion drive, but it was still painfully slow loading Word especially.

Welcome to the Apple Support Communities.

I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as seen here: https://discussions.apple.com/docs/DOC-250000211. We will then examine the report for issues in your Mac.

Cheers,

Jack