why is my lightroom so slow

I see you are using Classic so I will get your question moved to the correct community. I’m thinking this slowness must be to do with the upgrade of Mac OS. There are a few compatibility issues (see link below) although export was not one of them.

v8.4.1 was never designed for Catalina.  I’m sure Adobe will look to address these issues and any others that arise, in a future release. You could try trashing the prefs which often helps.

https://helpx.adobe.com/lightroom-classic/kb/macos-catalina-compatibility.html

To re-set the prefs quit Lightroom Classic. Then hold  down the two keys Alt(Optn) & Shift while starting Lightroom Classic. Timing is critical, so keep the keys pressed for a couple of seconds while clicking the doc icon or other shortcut. When you release the keys you should get a pop-up dialog (see image - click to enlarge) and you need to click the Reset Preferences button. The application should then launch.

It will help us to answer your question if you could provide the exact version number you have installed and details of your operating system e.g.
• Windows 10 v1809, MacOS Mojave (10.14) Catalina (10.15)
• Adobe Photoshop Version: 20.0.6 20190724.r.80 2019/07/24
You can find this from your application menu:
Help >> System Info

When you ask a question you need to provide some basic information
-Forum quick start https://community.adobe.com/t5/Community-Help/ASK-Forum-Success-Guide-Efficiently-using-the-forums/td-p/4791542
- https://community.adobe.com/t5/Get-Started/Questions-you-need-to-answer-to-receive-better-help/td-p/6199470