I've created an ARCGIS Online map and embedded on the website. It is really slow loading, sometimes also getting the message "The map is not available," (and then later the map would still load). It is happening in Firefox and Internet Explorer. Any ideas? here is the link: http://keepcaliforniaspromise.org/map-of-campuses
Your map is a little on the large size, with the amount of data in it. It's 3Mb in size. All the data has to get downloaded to the web browser and drawn by the browser. I suspect it's the polygon layers that are slowing it down. I'm guessing that the data has a lot of vertices. My suggestion is to generalize the data to minimize the number of vertices that aren't needed. Also different web browsers have different abilities in drawing features. Internet Explorer is the worst, and it doesn't surprise me that it won't draw the map. Here's a post that describes this: