Just some notes.
FPS alone no longer plays a large part in determining performance issues as rendering per frame is not done how it use to be as in how long does it take to render out a single frame. It can tell you something is wrong but can not tell you what it is that is causing the performance drop as part of the total number of rendered elements which is based on the refresh time of the single elements.
It could be lights, it could be shadows, it could be complex shaders. In other words it could be just one thing alone but as part of the total time in ms to refresh the render, based on the numbers, it’s a best guess as to the cause and what is required to solve the problem.
As part of the process it’s now best practice to complete the project and then record a profile of the project in real time and then play back the profile and step through per frame and see how the various elements are rendered as a total sum of the refresh rate.
The process.
By process of elimination you can see the result as to changes made as to elements that clearly, with out guessing, as to the impact of doing things like changing lighting types.
Also give forward rendering a try as well