well, SSAO will always lower your framerate. It require many calculations.
To optimize, do what Paul and Ali said, but i belive that lower scene tris-amount also might help(since it need normals?)
In your place, i would also rather look WHY your game work in low framerate in general.
Most important part would be Geometry amount, how many you have of them? render calls and tris-amount and custom-code was always most fps consumable part for me.
When I was starting on jME years ago, poorly written Controls and AppStates used to be source of slow fps due to my misunderstanding of how they work. Analyzing my code with profiling tools like VisualVM helped me to find and improve problematic code snippets.
Also, per-pixel post-processing effects will slow things down a lot more if you are running at 4k resolution versus 1280x720, etc… or with AAx16 enabled.