I'm looking towards using viewflow to design a workflow app for my company. I would like to know about performance constraints of using the non-pro version of viewflow. Besides this I'd like know if using the unpayed version I can change the default front-end that comes with viewflow and if so in which direction should I start looking to do so.
mm, there is no pro/non pro performance specific constraints in the viewflow. Viewflow PRO adds additional functionality to the open source version.
Fo the customization options you could check the viewflow cookbook - https://github.com/viewflow/cookbook (custom views and custom UI samples)