I have a -spd Neph myself, only that she's +HP instead. Gave her Fury as well as a Speed refinement for her Slaying Lance, and that has worked well for her.
Should fodder Wrath to Lyn if you have her, unless if this is your only Nephenee then I can suggest just going for a speed upgrade with Fury and a +3 speed seal like silveraura25 suggested.
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