I'll never understand how on earth Windows Defender tops performance tests. It's horrendously slow, particularly on large EXE installers and EXE files in general. Yet somehow it beats avast! which is light years faster. It makes no sense and never has. Literally every solution that I've tried was faster than Windows Defender. And I'm basing this on 3 different systems ranging from really crappy netbook with little ram and eMMC storage to mid range Ryzen 2500U with decent RAM and NVME SSD to a powerful desktop with everything to spare 4 times around. And yet it behaves the same on all 3 systems and is nowhere fast as avast!, Kaspersky Cloud or Bitdefender, yet it's somehow with them in the 6 score category. Ugh?!
EDIT:
It's also funny to compare all the above mentioned products to Windows Defender on "website performance" where avast!, Kaspersky and also Bitdefender do proper webpage scanning and analysis where Windows Defender just checks the URL and that's it. There is nothing to slow anything down, but also doesn't really add any protection value.
You're comments are mostly accurate but not fully. Just looking at the value 6 is the wrong way to look at things here.
If you check the details of performance test then,
Slowing-down when launching popular websites: As you stated, as WD don't have web filtering, it's obvious that it's going to be fast here. So full marks here
Slower download of frequently-used applications: Here again, WD don't do web filtering so it doesn't impact the download speed. The downloaded files will only be scanned after the download is completed so full marks here as well
Slower launch of standard software applications: Here it's about launching applications and WD is generally fast here. Click on your Firefox and it will load it as fast as the fastest AV out there. So, full marks here as well. Though in my own experience, some apps open the slowest with WD eg: Telegram, IDM, Everything and some open the fastest eg: Any browser, Steam, Revo Uninstaller. I would like to know what apps AV-Test actually opens to perform this test. If they disclose it anywhere then please let me know. But on most people's system, for common apps it loads fast
Slower installation of frequently-used applications: This is where what you pointed out comes into play. WD is definitely the slowest in this case out of every AV I tested personally and the test done by them also shows that. It is also known from experts to semi-expert users and even Andy Ful have mentioned this many times in the forum. WD is slow at accessing exe files as well as installing them. So thumbs down
Slower copying of files (locally and in a network): Now this one I'm not so sure about. I've seen WD not impacting copying speed of large files like a 500 mb installer but it slows down copying speed if the files you're copying has many tiny files in it. Copying Firefox profiles is always very slow on my system with WD. I guess, AV-Test, tested large exe, media, etc similar files for testing copying speed here so the outcome turned out to be good for WD
I think since app installing is something a user don't do regularly, they don't prioritize this outcome over other common actions like web browsing, launching applications which we do everyday. As WD did well in the common day to day tasks, I can clearly understand why and how on average it got a 6 out of 6 in performance. With 6 on protection with 100% detection, 6 on usability with 0 FP, this is one of the best results WD ever achieved
But then again, tests are subjective, AV's may behave differently on different systems based on many things so one shouldn't fully rely on these tests only and use whatever floats your boat