My Macbook has slowed down dramatically because of this update. What can I do?

You would have to find the cause. Someone in here could lend you a hand, but you should describe the issue more precisely. The following support link might get you started.

If your Mac runs slowly - Apple Support

Are you running any VPN, Anti-Virus, or Cleaning apps?

EtreCheck is a simple little app to display the important details of your system configuration and allow you to copy that information to the Clipboard. It is meant to be used with Apple Support Communities to help people help you with your Mac. It will not display any personal info.

https://www.etrecheck.com/

Pastebin is a good place to paste the whole report if you capture the URL while there…

https://pastebin.com/

Workable but harder for me to work with...the Note tool on the bottom of this editor's toolbar, as shown in the image, to copy and paste the output from EtreCheck. In a Reply before you click post, look for this to add longer texts..