bbl

No problem. Yeah as long as you dont need mobs spawning on all 20+ floors or whatever, a couple spawners would work just fine.

In short, No. No structure, regardless of what kind of blocks are used, would slowdown a computer. What would cause slowdowns is if other elements were at play, such as TNT explosions, massive mob spawning, ect. Non block elements basically are what cause slowdowns. I'll go ahead and ask this now, do your arenas use spawners as part of the design? I'm not asking because of potential slowdowns but because the design is going to limit the number of mobs that could be spawned.

In minecraft all the blocks are there as long as they are within rendering distance, within (x) number chunks radius are always generated around you. Because the game renders in chunks of 16x16x256, the entire chunk is rendered as long as it's close enough, not sections of it. Because of this the game uses only your X and Z (Left/right and forward/backward) coordinates to determine what chunks to render. The blocks render regardless of visibility so long as they are within your render distance, in addition, the chunks generate 65,536 blocks whether or not they are solid blocks, as air aka sky blocks occupy all the seemingly empty space that we walk around it. Easy version: As long as your tower isn't vastly wide or long, the entire tower would render because the height isnt limited by your render distance. So the sky level blocks would be rendered even if you were on bedrock level. Also, the number of visible blocks in a chunk doesn't affect performance because visible blocks simply replace invisible blocks, making the block count in a chunk the same no matter what.

I would be here to argue about it if need be. But the summarization of my argument would be 'Really? It's a snowball... Stop being a victim'