Upgrading my MacBook Pro 2017 to Catalina 10.15.5 caused my Wi-Fi speed to be cut down to almost 0mb. All other devices (iPhone X, iPhone 8, Dell Windows laptop) maintained top speed, so it was not an issue with my ISP or Wi-Fi router.
I tried removing plist items in System Configuration, rebooting PRAM, rebooting Wi-Fi router, changing network location, etc. but nothing worked.
Deleting the Wi-Fi service (go to Apple menu - System Preferences - Network and see list on left side) and then re-adding it back in solved the issue completely and it only took a few minutes. Tip - Click "Advanced" at bottom of Network screen and take screenshots of each tab (TCP/IP, DNS, etc.) before deleting Wi-Fi service so that you can re-add it correctly.