For me, I found out (after trying just about everything I could find on the topic on the Internet) that the problem was with Bluetooth and namely a Microsoft Bluetooth Mouse. As soon as I removed/Unpaired it on Bluetooth Preferences the Wi-Fi speed came back and web pages loaded fast again. Re-adding the Bluetooth mouse and everything went to a crawl again, remove it again and everything speeded up.
I did not have to restart Safari or the MacBook, although I did shut down and start up the MacBook again just to make sure it still worked and was not a fluke