As for multi-core. Never used it. Never had problems hosting without it.
But then again, EU was the only version with "Can you host" and "host list" threads. I don't know why, but EU players tend to have really really bad PCs. Like potato\toaster tier PCs.
Maybe try lowering all the settings to extra low and close all the background processes?
I have heard that multicore causes horrbile frameskipping/stuttering for the people connecting to the host.
In the cases for staff Evie it can be impossible to charge up her spells because the focusing just gets canceled/skipped partway through and you end up just casting pointless spells.
For others you end up being unable to block or dodge properly as your character ends up skipping through animations.
Can anyone confirm this?