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I had the same problem after upgrading my laptop a few years ago, and honestly it was a pain to diagnose at first. My speed dropped from 100 Mbps to like 20-30 Mbps for no apparent reason. Before you reinstall everything, you should check a few simple things that fix the issue for most people.
First, it's often network drivers that are lagging behind. Windows 11 needs the right drivers for your network card, and sometimes the ones installed by default aren't optimized. Go into Device Manager, find your network card and check if you can update the drivers (either through Windows Update or by downloading directly from the manufacturer's website). A lesser-known thing: disable "Accelerated Data Transfer" in the network driver properties if you see it, that can really mess up your connection. Also, Windows 11 has power settings that can reduce the network card's power to save energy. Go into advanced power settings and set everything to "Maximum performance".
If that doesn't change anything, check if you have apps eating up your bandwidth in the background (Windows Update, antivirus, cloud sync). Open Task Manager and look at the network tab. What's your connection normally (fiber, ADSL?) and have you tested on another device to check if it's not your router acting up?
It's not necessarily Windows 11 itself that's killing your bandwidth, but rather what's in it or what activates by default. First thing you shouldn't do: reinstall Windows thinking it's gonna fix everything, because you'll just waste time if it's a configuration or driver issue.
First check your network drivers. Windows 11 often installs generic versions that are far from optimal, especially if you've got an older Wi-Fi card. Go to your network card manufacturer's website (Intel, Realtek, Qualcomm, whatever) and download the latest drivers specifically for Windows 11. While you're at it, also disable the useless stuff running in the background: check your network settings to see if there's a VPN activated without you knowing, or if Windows Update isn't eating up all your bandwidth in the background. I'm saying this because once I had a friend who was tearing his hair out over his bandwidth, and it turned out it was just Windows downloading updates constantly. So also take a look at Scheduled Tasks, you can see what triggers every hour.
If that doesn't change anything after that, try plugging your PC in with Ethernet directly and see if the problem's coming from your Wi-Fi or the rest of the system. Often people think it's Windows when it's just the Wi-Fi connection that's gone bad. And it's not dumb either to restart your router, sometimes that fixes weird stuff.
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