Appdev has smooth scroll enabled while Squash is not. While Appdev’s scroll is not super smooth, it works well for most situations considering the natural scroll speed of an user actually visiting the website (as opposed to someone who is just testing it for smoothness of scroll by doing vigorous scrolling with unnatural high scroll speed ). It’s much better than the natural jagged native scroll of Safari and Chrome. Parallax backgrounds are best experienced with smooth scroll enabled.
If you like the native scroll better (the setting used for Squash), you can disable smooth scroll for Appdev by going to the Theme Options -> General tab and checking the checkbox ‘Disable Smooth Scroll’. I hope you are testing this in Chrome/Safari and not in Firefox. Firefox native browser scrolling is quite smooth and hence you will feel Squash is better. You must test and compare the two themes in Chrome/Safari before you decide to disable smooth scroll in theme options panel for all browsers. Hope this helps.