Slow internet on only 2 out of many devices

Hello TechRepublic community

I have a problem with my internet, which started to happen only a few days ago. I will try to be clear :

Basically at home we have two desktops, one laptop and a couple phones connected to internet thanks to WiFi. However, the bandwidth on MY desktop and phone have became very slow while my sister’s bandwidth is as fast as it was before!
I was wondering why it only happens to my two devices suddenly. Our router is still in the same room, my PC was scanned, I even tried to use a Ethernet cable…

Any help please? Thank you!