It’s a case by case basis, and there very well could be something wrong with the servers, but unless more people report this issue for a specific server, Blizzard will just chalk it up to ISP issues.
That’s also why I started off by saying people need to be specific and not just say “I’m having lag” for example. Hopefully more people chime in and say which server they’re connecting to.
I haven’t had any issues myself, but I play on US-West.