Diana has passive combo rate up (according to JP wiki 10% DA, 5% TA), +20 BG to self on burst and instant burst with 7T cooldown. Calling that slow is kinda weird to me.
Light Athena does not have any BG generating methods aside from getting hit (using cover), that is not fast, especially for light. Nobody's saying here that she's bad, she's very good tank (it's Athena after all), but she is indeed slow.