When developing or testing on Heroku, it is common to run a limited number of dynos, often only one.
Unfortunately, this will cause timeouts when you try to generate a DocRaptor document on your Heroku environment with assets loading from the same environment.
One dyno is busy trying to generate the document on our servers, but our servers are calling back to Heroku to fetch the images and stylesheets. Heroku makes us wait to get the images, because it's waiting on the document to finish generating. This results in a timeout because both systems are waiting on each other.
Heroku's documentation also describes this problem, but the solution is simple and straightforward. You just have to add another dyno or two (the more you have, the faster your document will load). If this is a development Heroku environment, you can shut those dynos down after completing your DocRaptor testing. If it is a production environment, you will need to keep these dynos to ensure that DocRaptor functions properly.