@SirriS_RU posted a workaround in https://forum.thesettlersonline.com/...-please/page12 so that you can still use the old client. It works for me and at least one other poster and gave me vastly improved performance. I'm reposting it here for visibility, with some clarifications and minor simplifications because I heard some people say the original post was too complicated:
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1. Find the game client folder where the file "The Settlers Online.exe" is located. It might be C:\Program Files (x86)\The Settlers Online or it might be %localappdata%\Ubisoft\The Settlers Online (you can go there by copy-pasting that into your file explorer). If you only see client.exe but not "The Settlers Online.exe", it's not the right location.
2. rename the file The Settlers Online.exe to anything else you like, e.g. The Settlers Online old.exe.
3. rename the client.exe file (or client_real.exe if you used the highlight) to The Settlers Online.exe
4. download the[removed]file and place it next to The Settlers Online.exe
5. install the Tampermonkey extension: Chrome [removed], Edge [removed] , Firefox [removed]
6. in your browser, in the extension tab, activate the Tampermonkey extension if it's not activated already.
7. install[removed] in the browser via Tampermonkey
8. on the main page of the game, press OLD CLIENT
Every time there's an update, @SirriS_RU also updates the file [removed]in step 4 and then it works again (same link, different file). So if you had this working and now it's not, then just re-download the file from step 4 and it'll work again. If you want to get this working for the first time, the steps above are still valid.