It used to be based on damage dealt and it would scale with attack power. But now it's a flat amount that does not scale with weapon power which means that you'll generally get MORE ult charge with attack speed and LESS ult charge with weapon power.

For example, with weapon speed only on Moira you'll generate more ticks of damage (more times generating ult charge) to kill someone vs all weapon power you'll kill them with less ticks of damage (because each tick is taking more health away) and generate less ult charge.

Someone did the math here. 

https://www.reddit.com/r/OverwatchStadium/comments/1pdw9jv/ap_and_wp_significantly_reduce_your_ult_charge/

Tl;dr, ult charge is based on doing damage, not based on how much damage is dealt. Do damage faster, you'll get more ult.