Mihawk not being faster than Kaku is like saying Gold Roger wasn't stronger than Luffy is now.
Again, I don't see how Mihawk's speed is related to his prowess with the sword. I'm not saying that he isn't faster than Kaku, I'm just wondering why Mihawk being a better/stronger swordsman automatically makes him faster than Kaku. As we saw with Zoro vs. Kaku, you don't necessarily have move faster than your opponent to defeat him.
Also I don't really follow your logic here, unless you're talking about Gold Roger's pure physical strength compared with Luffy's. In which case, again I don't see how you can automatically say Roger was stronger than Luffy is, since we haven't seen any direct evidence that shows how strong Roger was. It's possible that he was a DF user, and just really knew how to use his abilities really, really well. What's the probability that Roger was (physically) stronger than Luffy is? Well I have to say that it's pretty high, but it's not 100%.
Now if you meant that Roger's overall strength was higher than Luffy's current strength, I will have to agree. However, then you can't use it to support your argument in this way. Speed is only one part of one's overall strength. Consider these stats that I've tentatively applied to Zoro, Mihawk, and Kaku. If you don't like them, just imagine they're 3 random people then.
On a scale of 10:
As you can see here, if you go by total strength, Mihawk is above both Zoro and Kaku, but taking speed alone, Mihawk is slower than Kaku. In this way, there exists a scenario where Mihawk can be stronger than Kaku, yet not be as fast because he makes it all up with swordsmanship and other things.