It's 90% of your base casting speed (You have 100% in robe, and -10% that means that 90% of your base cst.spd in Light armor) so is not a plus. Skill has misstype as a lot of Lineage 2 skills when devs translates it from Korean to English. Also you won't notice an increase of atk.spd because Orc Mage has Magician's Attitude that increase atk.spd wearing Robe Armor (and it gives 25% too).
So if you're playing as mage go for MJ set and NM robe helmet.
They will receive a lil boost but i don't know if will come with this new patch.
Boost is a buff called Mass Servitor Boost. This is the link from KR server. https://lineage2.plaync.com/board/l2awknupdate/view?articleId=223623