It can be really frustrating to debug an issue like this. But, the fact that your speed tests at the time were consistent suggests it might be an issue deeper inside your ISP's network. On the other ...
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It can be really frustrating to debug an issue like this. But, the fact that your speed tests at the time were consistent suggests it might be an issue deeper inside your ISP's network. On the other hand, you also said things got worse when other people's devices got closer to you, which suggests it's a local WiFi-specific problem. That could be any number of things, so let me just suggest a couple things to look at. First, do you have any wireless audio devices? In particular, non-Bluetooth systems like wireless surround sound systems, a sound bar with a wireless subwoofer, or wireless gaming headset. These can all use 5GHz WiFi in a way that interferes with nearby devices. When other people's devices come into the area, it just squeezes the system even harder.
Second, are you playing on an Apple device? A mac of some kind (macbook, macbook pro, iMac, mac mini, etc.) or an iPhone or iPad? Apple does some fancy stuff between their devices that uses the WiFi hardware in ways that don't impact buffered streams (like YouTube), but can definitely impact interactive streams like Stadia. Location Services is the big one, but there are also some device-to-device protocols (things like the shared clipboard between devices, etc.) that come into play as well. This may only happen when multiple devices are "awake" and near each other.