I have a similar issue when I am visiting my parents. Despite having 30 mbps upload at my home, I cannot get anywhere near that when trying to access things from my parents house. Not just Plex either, I host a number of services. I've tested their wifi and download, and everything seems fine. I can also stream my Plex just fine from my friends places. I've chalked it up to poor (or throttled) peering between my parents ISP and my ISP. I've been meaning to test it through a VPN next time I go home.