Wich Julia?

I'd personally run +Spd/-Atk, mine is +Atk/-Spd and although she hits hard 23 base speed is rather ridiculously slow and might just get doubled unnecessarily by most of the mid-tier speed tier units.

Neutral seems the best to me. Julia never doubles, so is best as an enemy phase res tank when Rein is around, or a powerful one-hit Nuke like Lilina/Sanaki when he isn't. Neutral speed and Fury 3 should put her around that 30 mark after the merge (no useful SI), which is enough to deny doubles from the things that matter to her (sword units, for example, will double her to hell and back, but that doesn't matter as if she's being initiated on by a sword unit you were screwed anyway).

+hp is ok for a tanky unit, but -spd may be a bridge too far. -atk is just the worst for her.

Neutral one could be the best in my opinion. I think -spd hurts her as then she can't double Reinhardt with horse buffs and she can't always one-shot him.

On the other hand, if you go with +spd/-atk, it's not that bad either, but I'm not certain how much the -3 atk would hurt her. I pulled a +spd/-hp Deirdre (33 atk/31 spd) and I don't feel like the low (compared to my most used mages) attack is hurting her ability to kill blues imo.