jobel - the largest image is 640 x 360 (only one of them - the BG). I have probably about 1000-1100 objects (600 are invisible menu items) and only a few (maybe 30-40) are on the screen at a time. None of them but the player has any behaviors (platform) and one tilemap that has solid. Layout is about 4500 x 2600.
It all depends on what your game is doing. I have far more objects and an unbound layout and my game runs at a steady 60fps. My game is a decent size, I have about 3500 events and the main layout also loads at least 10+ large sprites over 800x800, they are not always visible. The player character has around 48 frames of animation and currently it's a good 300x300px (downsized in-game).
The only time I had a frame drop issue, was when I didn't cap out one of my spawners. I had been creating too many objects that were never destroyed (only if the player was near them).
Not sure what to tell you, except that I've always tested gradual.. any time I add something new, I would check to make sure the game ran okay. When I had my one frame drop issue, I noticed it right away after I implemented it. So I knew I had a problem that I needed to deal with. Adding a cap system worked great, and the player never notices it.
I think you need to find exactly what is causing it. What are those non menu object doing? there's about 600 of them. Do they have lots of frames of animation? try eliminating a group and see what happens. As long as you can reproduce your frame drop, you can always find the culprit, just gotta do some investigative work...