If you stutter badly for a while, then things eventually settle down and start being playable, then your primary problem is the time it takes to get textures from disk into RAM. That is a function of disk I/O, thus an SSD would benefit you the most.
Thats kinda true but the game will render even if you don't have textures, I am sure you have seen many times when you log in you only see pets for a bit then you see character models. Pets are obvisoly loaded first as they have few options for gear etc. Even though the game has no textures for the character models yet it still will render the frames without them.