Have you tried to reinstall your wifi-drivers?

How would I go about doing that?

Now the website has gone back to normal access, which makes it harder for me to test if any fix actually fixes the problem. This seems so random that I feel like it could be a signaling issue on a cable somewhere outside my house.

Hi Pietro, welkome on our community. 

How unusual that you're experiencing this randomly on websites. An external cable outside your house wouldn’t explain this, as it would affect your entire connection.

On which devices do you experience this? A computer/laptop or phone? When the website loads slowly on, for example, your laptop via WiFi, does the same happen on your phone via WiFi?

By the way, you can schedule a callback online (scroll down a bit), so you don't have to call yourself. 

Thanks for the reply. The problem happens on any device, be it a laptop, phone, or TV. I have only found 1 website where this issue happens, but I have verified that this website works perfectly on any connection other than my wifi. I have noticed a pattern in the ‘slowness’, where it usually starts somewhere around the day, and then goes away (as in, the site goes back to working fine again), late in the night. 

Can you tell us on which site this issue occurs?

https://pbogre.com, it has some private subdomain that provides media streaming, and this media streaming is the main reason why this issue is bothering me. Note that it is hosted by myself, and therefore not running in any server farm. For a long time I thought that the problem was on the server’s side, but the fact that it works perfectly consistently anywhere outside of my wifi at all times, and since it can’t be a firewall issue as it’s just much slower than normal,  makes me sure that it’s a problem on the wifi side.

Something that might be worth mentioning is that my flatmate noticed his wifi extender also only works well at certain times, and those times seem to be similar to the slowness experienced on this website. This could just be coincidental however, and remember that all websites (that i’ve tried) and general internet access is completely fine all the time.

So just to be clear, all websites work as they should except the ones you are hosting? How is this exactly being hosted? 

Does this also happen when you use a UTP cable instead of WiFi? 

That’s correct, and the websites i’m hosting work fine with any other connection. They’re hosted with nginx and accessible to the public. 

I have not been able to test using a cable instead of Wi-Fi, and I can’t now because the connection is currently working fine. Next time it becomes slow I can check.

Please do, because I can't really explain this issue to be honest seeing as other websites are working.