A lot of the time carousels can tack on lots of added bandwidth, both in the slider functionality as well as the images within the slider. Site speed is a large factor within SEO and has been known to make a site respond accordingly.
You mentioned you switched CMS's as well: did you pay attention to your permalink structure during the change? Were any 404's created. Did you 3xx old posts to the new ones? Sitemap?
All things worth looking ito.
