Having had a very sluggish Nexcloud experience for about a year, blamed my rather low-spec server for it. Then I stumbled over a rather sneakily introduced (anti-)feature for Windows, called “Task Manager Efficiency Mode” (W11) or “Eco Mode” (W10), which uses EcoQoS to temporarily throttle unused (or are they?) processes. It is implemented more as an API feature, so app developers can choose to make use of it in their apps (which Chromium does, unfortunately). There is also no way to globally disable it in Windows.
You can temporarily disable it in Task Manager, or you can start Chrome (or any Chromium browser, supposedly) by appending “–disable-features=UseEcoQoSForBackgroundProcess” to the launch command.
In practice, you want to open the properties of your browser shortcut and make the Target value look like this:
"C:\Program Files\Google\Chrome\Application\chrome.exe" --disable-features=UseEcoQoSForBackgroundProcess
And that should be it.