I dont have a super clear answer for you because I have always had problems with iOS being way slower than Android. I do have a couple of suggestions though… Check your Appium version and Java Selenium version… iOS became very slow after upgrading to Appium 1.8 but then I got my speed back when I also upgraded Java-Selenium… here is a post about it
Also, in my experience, iOS has a hard time with xPath… using CSS names to find elements has made things faster…
I also got some speed by not using selenium’s implicitlyWait when running on iOS… Im having trouble finding the post where I learned that
I hope that helps a little… Good Luck!