I think your tinfoil hat is on a little too tight.

Did those ISPs you were on in the same area? If yes, then they are most likely on the same backbone, which, during peak times the connections can be slow.

Also it also depends on the sites you are visiting. People could be hitting the site you are trying to visit and the server is slow with responding to requests. So regardless of ISP the site you are visiting will be slow.

There are many factors as to why a site could be slow, hell, you might just be having a bit of bad luck and all the computers you used which were on different ISP were probably slow.
The router could be an issue, etc.

TL;DR: Learn how the internet works.