You can’t save to a filesystem on Heroku, it has no persistent storage, so what you’re trying to do can’t work. On Amazon you’d normally use S3. However:
Saving actual tickets is slightly ridiculous. Physical tickets are an abstraction, they exist so that a person can have a physical version of some data. They have absolutely no meaning in the context of an application which stores ticket data in a database. You should just generate a PDF version when someone asks for a ticket, you don’t store those PDF tickets.
Being slow could be to do with a few things, none of which are likely related to Mongo, and the cause will be something you are doing: saving/reading from a physical filesystem, making huge amounts of API calls in a very short space of time, inefficient JS on the front or back end, unknown anyway, you need to work through the app and isolate exactly what is slow: JS has fairly good debugging tools, use them.