It's not as simple as that. If all you care about is DPS on bosses then yes, get a sky ski. The exact weapon attack it needs to have to outdamage a slower spear can be found out by using a damage calculator (for example this: https://royals.ms/forum/threads/damage-calculator-all-classes.123479/#post-704000) However, if you're grinding and maybe not using Apples/Stoppers much, then a slower spear can be better, especially if it allows you to 1-hit something you'd 2-hit with the Ski.
Faltizan could have been a good alternative if royals wasn't so abundant with att pots but using stoppers /apples in boss runs is such a norm now. Sky ski is better in 90% cases unless you are determined to slaughter Bigfoots with SI mule. That 1 little difference in speed creates bigger gap as you get stronger.
