Why is my latency so high whilst streaming to my PC on the Xbox App?

Like I said before, it's far from an ideal setup for what you want to do here.

So I can stream to Twitch perfectly fine but not from my Xbox to my PC? I don't understand why that's the case.

I'm at student accommodation,

That's far from Ideal, too many others sharing the same internal network.

It checks the network and says it can do high but that doesn't mean it can, it doesn't know how many others there are on the same network, which is the reason it's so poor.

That kind of network just isn't suitable for streaming games, certainly not to the quality you're after.

Just a note.

Your internet connection speeds provided by your ISP doesn't count when you're streaming to other devices on your own network. What counts is the quality of your own network.

I'm at student accommodation, I imagine that the quality should be great.

Would the Xbox app still tell me that I can stream in High quality if I couldn't?

Just a note.

Your internet connection speeds provided by your ISP doesn't count when you're streaming to other devices on your own network. What counts is the quality of your own network.

So I can stream to Twitch perfectly fine but not from my Xbox to my PC? I don't understand why that's the case.

I'm no expert here but there's probably latency compensation for streaming to twitch. What I mean is, it doesn't need to be as quick to process the video from your console to the twitch servers. After all, the users watching the video feed aren't playing,
they're just spectating. While you're live, they don't see what your seeing at the exact same same time you do, if that makes sense.

With game streaming from one system to the other, timing is everything. Even on a fast connection, it's still not quite the same as playing on the host machine, there is a latency issue. Your controller movement has to be processed, the video frame/s sent
to the client (the PC), the image processed by the PC's hardware and sent to its display.

This processing, plus heavy network traffic, is the cause for the latency issue you're experiencing. Twitch users won't see this, the frames have already been processed, they are just watching the feed.

Perhaps you should test this when fewer people are using the network?