Actually I don't see how this armor would slow Geralt down or hinder his movement. In fact long, wide clothings are pretty easy to move in; think of the outfits (called "gi") that Judo and Kendo fighters wear.
From my personal experience (many, many years of Karate, Ju Jutsu and Kali) I'd say: A good robe that's going way down, close to your ankles, may be a good thing to have, under some circumstances. If tailored correctly it doesn't restrict you, and sometimes it may even conceal what you do with your legs.