One very common issue we see is that you may have loaded a large number of fonts to the template. While it is nice to offer your customer a lot of options to choose from, most will not want to scroll through dozens or even hundreds of font choices.
Each font name has to be loaded, then we need to load the URL's for each of the font previews, and then raster the text on the template. For templates providing hundreds of fonts you can understand how this would significantly slow down the loading of the template.
Other possible reasons would be the number of pages in your template, or the number of fields you have on your pages. The more you need eDocBuilder to load in order to provide the template preview on the screen, the longer the process may take.