Light slows down in matter because it keeps stopping every so often when it collides with particular bits of matter. It keeps getting tripped up, if you like.
Antimatter is just as willing to trip light up as matter.
So light passing through a block of antimatter will slow down. Gravity attracts two lumps of antimatter together exactly as it would attract two lumps of matter together. In that sense, matter and antimatter are exactly the same. Light speeding up would also create a lot of problems in other areas of physics, too, so its just as well that light behaves in the same way when interacting with matter as with antimatter.