Check your video setting is it running 16bits color ? if so change it to 32bit color. :I also encounter this issue whenever set my crappy laptop to use 16bits color to make my video chip free up its resource to run the game.
PS : if ur video lagged using integrated video chip like Intel HD graphics , try to zoom in ur camera & ur char should move smoothly again . zoom too out & the screen will look laggy.

Note : Since u left-click the mini-map to move , i assume u are using the default client . go to this URL
http://dl.dropbox.com/u/18779594/salem/updater.jnlp to download the Ender's Client launcher instead . Once u use Ender Client , u will never log in without it.
