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Yes, there is some truth in what your designer says, adding any additional images and media to a page will slow the load time, but that doesn't mean you shouldn't do it. My advice to you would be this, if adding a slider to an internal page enhances the user experience (UX) then add it to the page, a well optimised image slider will not impact the load time of a page noticeably to a user. Once your site is designed, you may want to consider running it through the Google PageSpeed tool, the tool will score your page and generate suggestions to help you make your page faster https://developers.google.com/speed/pagespeed/insights 

Thanks for your comment.  The home page for LancasterPA.com isn't in the top 50 for Lancaster Lancaster, PA Lancaster, Pennsylvania Those would be our main keywords.  I'll re-think our current structure, but I'd welcome other suggestions. Eventually I'd like to move our other site (www.800PaDutch.com) to this domain, but I first want to make sure there isn't an issue with the home page ranking well.