Hey! There are no servers that Thunder proxies to, it sends everything directly to the instance you are on. The main issue with slowness will usually come because the instance you are in is experiencing server issues or timeouts because of too much traffic
If you are not logged in, it will default to the lemmy.world instance, which is currently experiencing a lot of delays (at least when I am using that instance)
Which instance are you on? You can try a different smaller instance if you have an account elsewhere and see if the slowness still persists. If it does persist, then it may then be an issue with Thunder and you can open up a GitHub issue on it!