Am I using up all your bandwidth when I use this?
Not at all, please feel free to install as many updates as you like. The updates themselves come directly from Microsoft servers. The website of course is hosted on my server, but employs Cloudflare edge caching, so you’re in fact rarely sending traffic to my actual server.
When you check for updates, the Windows Update protocol traffic is proxied through my server, so that it can be downgraded from modern encryption to something old Windows can understand. All the server currently does is pass through exactly what your machine, or the Microsoft server, sends, with no processing beyond that. This traffic isn’t small, but it’s also not really big enough for me to be concerned about either. It’s well below the monthly bandwidth limit I’m allocated by my host.
Why is it slow?
Windows Update is a complex protocol, and applying updates is an elaborate juggling act to ensure only the correct updates are applied, and in the right order. Performance problems are inherent to Windows Update, and aren’t specific to Legacy Update.
When you check for updates, Windows and the Windows Update server compare notes on what’s installed on your system, and therefore which updates are applicable to you. Because there are thousands upon thousands of updates, this is a very long, slow process, heavily tied to your CPU’s single-core performance, and hard drive/SSD read performance. If you watch Task Manager while you check for updates, you might see svchost.exe, wmiprvse.exe, and TrustedInstaller.exe (on Vista and later) using up an entire CPU core. This is the Windows Update Agent evaluating the configuration of your computer so it can let the Windows Update server know which updates it needs to see. Old PCs can take minutes to complete this stage, while a VM running on your modern laptop should fly through this in a few seconds. This process is heavily dependent on your CPU’s single-core performance.
While installing updates, you may feel a slowdown on lower-end PCs due to the volume of hard drive write activity. If you have a small amount of RAM in such a system, and a fairly slow hard drive, this can really hurt the system’s ability to use the hard drive as swap/pagefile space. This will clear up once the updates finish installing.