Generally your bulkier nuking mages like +spd Julia, Sanaki, RobinM, Camilla (Spring), and importantly neutral spd Ike, +spd Nowi and most of horse emblem sit around the 30 spd benchmark, although you'd want to factor in 3 spd from Fury 3 which most of these characters run in order to avoid crucial doubles by the faster characters
If your speed is less then 30spd then you need atleast a def or res stats of 35+ to remove this 2 hit penality.
See Hector, Xanders Sheena Frederick Julia Sanaki as example slow but have high def or res and are still strong in Arena.
Sharena on other hand let be real is ok not strong not weak . She is still fine Bulky 29 def enough as a lance to kill most of the red lords in game her spd block the most double in game but she will do rare double with a spd of 32 / 35with +3spd a slot or fury what should be really recommend to run to make her more bulky