Yaks are really fast, they just seem slow because they are boring. Make sure you are using your range void outfit, unholy book, best amulet you can afford, archer ring, etc. Max out that range bonus while still wearing void. I did this to 80 range.
Once you get to 80 range and have good defense, I would try fire giants. They have a good amount of health so you can stay on one of them longer than a yak.
If that gets boring, you could try soul wars. 100% of your game should be devoted to owning noobs for maximum experience. Experiment between void and regular range armor (karils/black d'hide/armadyl), sometimes you might prefer accuracy over more powerful hits if you aren't hitting often.