Network slow down

[subas113]

Hello i have a 2 computers connected to a router to split the internet from a modem, and when i use PC 1 everything is fine but when i connect PC 2 the internet slows down even when PC 2 is just in idle, and only whatching twitch streams or browsing if I try to download a file it have normal speeds(1mb\s +/- ) in both the PCs, i tryed to disconnect PC 2 from the router, and I could see a stream or 2 fine again in PC 1.

I have already run Malwarebytes Anti-Malware and hitman to look for malware but found nothing.

If someone can help or just point me to a software that monitor LAN network i would be grateful. Thanks