I know that the technologies are different, which one is supposed to be faster, with a better user interface response?
ADF can have great user interface response if used with performance in mind and utilizing Javascript inside your application.
Is ADF slower than FORMS because of the heavier interface?
No, both behaves the same under the same circumstances, ADF can be enhanced when you apply performance procedures and ADF also provide great user experience to the user.
What can be changed to increase performance on a ADF Application (Server?)
Consider reading this documentation
Are there any applications to perform benchmarking to the application?
Use JMeter with ADF, see a link for a demo, and another article as well.