Didn't really understand what was the problem that made you add spikes for the stairs. Couldn't reproduce any graphical glitch on my end.
Unfortunately, performance does go down the more complex a scene becomes.
Assuming you're using a Tilemap/Composite Collider combo, the only advice I could give is to separate your map into chunks (like the image below).
This way, the light only has to take the relevant vertices into account, instead of validating vertices far away and not in range.
I'm not sure if the light source in your image is your rock-character or that triangle-shaped fire (?).
If it's the latter (and your character doesn't cast shadows), it's a good idea to activate the Calculate Only Once check under the Optimization Settings. This will "bake" the light mesh on the first frame and you won't have to worry about performance anymore.
Hope this helps somewhat 💦
If you got more questions, please let me know 👌
