So I have a macOS Sierra version 10.12.6
it runs extremely slowly and takes forever to load websites etc. Even though it is in WLAN.
I haven't used it for 3 months and now it's just so slow.
Is the laptop just too old or do I just have to get it going again, so to speak?
eh yes
You just told us the operating system… What year is the device from? Is it a Pro or Air and what hardware has it installed.
To do this, of course, we would have to know how old your laptop is, what year and what the technical data look like. It would also be interesting to know whether it still supports version> 11.0
Ooops, where can I see what kind of hardware it has and such?
That depends on the hardware, not the MacOS version.
Sierra came out in 2016, a 4 year old Mac should be enough for the web.
https://www.macworld.co.uk/how-to/how-check-mac-specs-processor-ram-3594298/