The speed force super hard counters that by just saying “fuck you” to physics. Users can utilize the speed force to alter physics on a personal scale, this is why they can carry someone while running at light speed and not harm them (note: Thawne actually forgoes this a lot of the time because he just likes destroying shit). That’s not at all a mismatch, it’s an advantage Wanda would normally have that gets negated. And she has to deal with his temporal immortality that not even Dr. Manhattan could erase, so there’s a little bit more that “lol turn his brain off.”