Hi.
At present all the client buttons, when clicked, would start to do their actions only when you release the mouse button.
Try to click: a menu bar button, any crew skill, a talent tree element, a quick slot item, or even this "emotions" popup menu in the chat frame. Try to select someone in the raid frames.
This fact that all of them are bound on WM_LBUTTONUP instead of WM_LBUTTONDOWN is one of the main reasons why the game's UI feels so slow and unresponsive in general.
Please consider changing this behavior in favor of button DOWN wherever it is possible.
I think this should be relatively simple thing to do, which could greatly improve UX nonetheless.
Thank you.