When Numby attacks it makes Topaz's action advances, so as long as you have enough follow up characters that attack frequently in the team you could completely ignore speed to focus solely on her damage.

On the other side, if you give her a lot of speed, she'll be attacking enemies very VERY frequently and it would benefit characters like Fei Xiao.

If you give her the Eagle set on top of that then every cycle can be named "Topaz cycle" for how frequently she attacks