I have a Mac Mini(late 2012) 2.6 GHz Intel Core i7 16GB DDR3 macOS High Sierra and a Mac Book Air (2015) 8GB Ram 1.5 GHZ Intel Core i5 macOS Sierra. I connect to the internet on my Mac Mini via wifi.
All of a sudden like 2 years ago the internet became very slow on my Mac Mini. At first I though it was my ISP. Last year I got a Mac Book Air and hooked it up to the internet via wifi. The Mac Book Air gets 10 megabits download speed using the same internet connection I thought was slow but my Mac Mini gets 2 Megabits downloads speed with the same connection. My Mac Mini also has a higher latency compared to the Mac Book Air.
I thought, ok same connection different computer. So I reinstalled macOS High Sierra on my Mac Mini thinking maybe it was a virus and I am still getting very slow internet.
My next line of thinking was the builtin wifi card on my Mac Mini was old and outdated and maybe it was not able to take advantage of the new wifi router we got two years ago. So I'm thinking it's a hardware issue but I am not sure.