Latest versions of Attract-Mode include a handy plug-in by Oomek called FPS Monitor. This monitor can be turned on through Configure > Plugin > FPS Monitor> Enabled yes. In the same page add a key for turning FPS Monitor on and off, if you have not mapped "reload" to any key then you can add a key for reload here too. I found Graph size "Large" to be best.
Usage:
When FPS Monitor is turned on in settings, it will auto-run on start (I wish default was off) go through your layout and keep an eye on the FPS Monitor, at any place where you see FPS drops (Usually between 50-59) check the layout code and see whats causing the drops. You can also narrow it down by turning off artwork one by one. Thats where reload key comes handy, after any changes in layout you can press the reload key and layout will reload with new changes.
Depending on the reason you might need to convert png to jpg or 8bit png or re-compress through website like tinyPNG (thanks Enkak!). Or you might want to resize if the image res is huge, specially when you are using reduced res output of said image in layout. You might also want to use something like Transition.EndNavigation.
BTW Videos WILL cause FPS drops, best to do this with videos disabled. I don't know of anyway to minimize drops on videos right now, though in some tests I did wmv caused minimal drops then avi them mp4. But this is on Win* system might be different on *nix.
As Keil and Zpaolo mentioned in comments further down, multiple surfaces (specially nested) and heavy shaders can also cause slowdowns/ FPS drops, you can comment out one item at a time and reload/ check.