According to the forum entry, clients accessing an application from a slower site can impact the performance of clients accessing an application from a faster location. This looks to be due to blocking in the Parser code. It's related to sychronization that was added to fix another issue, ICE-2936

The Parser is not re-entrantly threadsafe as an application scoped instance, so the parse method has to be synchronized. It can't be made session-scoped because the parser is 140K/user. And it can't be request scoped because it takes 2+ seconds to load the tag rules each time the parser is initialized which is added to the render time.

That's the source of the inter-thread blocking.

We may have to load the tag rules statically in the constructor and then construct a per-request instance of the parser. This will require a little bit of breakage, because the parser is a member variable of the ViewHandler, which is an application scoped entity.