The Performance of Text Vs Image isnt SQE fault this time Blame DirectX and GPU methodology. Icon or Text does not matter what does is a smooth server , less load and a friendly compact interface .
Take people that like Overhead Playername+Mobsnames . The amount of data needed to display these is higher than people expect have these on in a large fate and watch your GPU,CPU cook.

Text Rendering had always been costly in any 3d environment.

As for Text in a image based enviroment its a case of Number of Render Calls + Render States if you want to pack an image of every line of text and mouse over state and in a good quality font you will hit bottlenecks of Processes or Memory usage. Plain Line Text will Bog the Client down its a 3D realtime app not a word processesor that can cope with 10 fps.