I'm having a similar issue on Ubuntu 11.04. I think it happens when Sublime thinks I am dragging the tab off the current window. It then opens a new window to receive the tab, and that process seems to take a lot of time.
However, I think the real problem here is that Sublime thinks I want to drag the tab off the current window when in fact I am just trying to drag it within the current window. It seems way too sensitive to vertical "drift" as I'm dragging the tab. Unless I concentrate really hard on keeping the tab within the tab area whilst dragging, Sublime will decide to open a new window.