She only really does damage with her A3 (Heartseeker). That's where her money is made, so to speak as it is a max HP nuke. 

Should I disable any of her other skills so she will get to A3 more often?  If I can add more speed will that improve the chances of her getting 1 more big hit in a battle?

She'll prioritize the A3, so disabling other skills won't have any effect. Speed helps, sure, but your team shouldn't really require her to use it more than twice, at most, and having more speed won't really help you get that.

You'll almost always want her to be 151+ speed (to outrun the spiderlings) and typically with a reset for her A3 like Renegade or Kymar. 

I dont understand to reset her A3 like Renegade or Kymar.  More details please.  

You use a champion like Renegade (who uses her A3 to lower ally cooldowns by 2 turns) or Kymar (who does a full reset of allies cooldowns) with his A3 after Coldheart uses her A3, thus getting Heartseeker back faster or immediately. 

You'll frequently see a crowd control option like Achak/Miscreated Monster + an AoE HP burner with 2 Coldhearts and a reset as a speedy Spider 20-25 farming team. :)