These forums are hosted in Japan, and they feel pretty "lag free" to me. But I guess it's all a matter of perception.
Lol? Forums are irrelevant when it comes to lag- you only interact with them when you want to load a page- your ping will only impact the time it takes before the page begins to load. Beyond that it's not going to load any faster or slower than any other web page.

In a video game, you're constantly sending commands to a server and thus are constantly being affected by this delay. It's still a minor thing, but it matters more when it's constantly coming into play than when you're on something static like a web page.

People always think "JP Only" means "Japanese Person Only."
Actually, I don't think anybody thinks this. Where did you get this idea? That being said, Japanese isn't very widely spoken outside of Japan, so there isn't a whole lot of difference.