sounds like your ISP is artificially slowing your connection down
try connecting through a VPN and see what happens. I'm willing to bet your speeds will jump back up to what they used to be (ISPs bottleneck "known" ports that certain programs like steam use, those port assignments are different on a mac, so they probably weren't bottlenecked). I have the same problem at my house, my provider artificially limits my download speeds with torrent programs and programs like steam as well as netflix and youtube. so i run my system through a home VPN, fixed the problem on the spot.
vpns are incredibly useful for many reasons, so it would be worth setting one up even if it doesn't resolve your problem. that said it sounds like this is the source of your issue. That or you did something silly like add a 2nd router and now have 2 routers on the same network