There are no provided numbers so really cannot say
Maybe tell us more about your scenes; give some video maybe with debug draw so we can see how many drawcalls/verts etc. Could also be an issue with shader setups or other material/object settings or complexities.
Games require optimization to run real time, performance tuning is a part of game design and there is no getting away from that, certain engines require different optimization rules that you would apply during object creation etc, these would be explained explicitly or implicitly during the many asset creation tutorials currently available. In essence, you should test in increments for performance. If you notice a performance drop after a certain action, change the action and see what is better (like material/object properties for ex).
Alternatively if you are already familiar with typical optimization techniques for the engine (merged meshes or w/e) , then you would typically do periodic performance profiling with a tool such as brofiler etc to see what performance bottlenecks are holding you back. This can get seriously technical in a flash so you need to know some things to be able to use these kind of tools like what the data actually means.
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