singe object or multiple objects for game scenery

I’ve read that blender is better at handling less objects with lots of polygons that handling lots of objects with less polygons. Up until now I wasn’t concerned with this aspect but, after creating a scene to test my game project I noticed an alarming drop on the fps (25 - 30 fps less).
I made a simple test, I created a scenery with some modular pieces and a few lights, and then I made the exact same scenery but I joined all the meshes into a single object. The result was exactly the same.
What is best for making the levels of the game? Lots of modular pieces (each one an individual object)? A single object with a bunch of materials and textures?
What kind of witchcraft is involved into those AAA games that run at 59 - 60 fps on awesome looking landscapes?

You need a balance.

Several objects

Are better processed at frustum culling -> reduction of rendered faces
Are harder to manage due to the amount of objects.
Are more processing inefficient due to the larger scene graph processing, and object lists.

Large objects ignore the benefits of frustum culling.

… + other aspects.

Conclusion: do what works for you

Fair enough.
A quick question.
Upon further testing I’ve noticed on the profiler that, while running on a window about 1/4 the size of the screen at roughly 16:9 aspect ratio, 50% goes to ‘rasterizer’ but! if I change the size of the window (i. e. full screen or something larger than 1/4 the screen size) or the aspect ratio, then about 40% go to rasterizer and another 40% to gpu latency (and some fps drop).
I have absolutely no idea why, any clue?

GPU latency is, as far as I understand at least, time that the GPU is working after everything else is done (which is to say, when the CPU is done with its bit, if the GPU has to take more time to process stuff before the frame is finished, that extra time is GPU latency). It’s broken out into a separate category so you can tell that you can either cut back on graphical options to get more frames, or you can increase CPU processes like logic or physics without making the framerate even worse (since the CPU is waiting on the GPU anyways, so you have room to give it more work).