I'm well aware of that and I did the math while making the post. The idea was for it to be possible to spawn the boss fairly quickly if you were trying to, but not super fast by accident unless you're going on a magma cube rampage and simply don't know about the boss yet. The harbinger cubes are supposed to act as a warning, so that you aren't too surprised when the boss spawns on you, and hopefully make it clear enough for the player how to avoid spawning it if they are afraid.
This spawning sounds problematic. Even ignoring the
issue I mentioned above, 50 points times 20 seconds per point is 1000
total seconds to naturally enter this mode. That's 16.6 minutes. So just
about every day and a half, with or without player intervention while
also including the ten minutes of boss mode.
I said it ticks down every ten seconds, not up. That means you can kill magma cubes pretty often and never see the harbingers. The whole process of spawning it on purpose could be as little as ten minutes, but you could live in basalt deltas for dozens of hours and never accidentally spawn the boss.
It would be easy to accidentally spawn the boss, that's sort of the point. I'd want a player who doesn't know about the boss to discover it eventually. I also made sure it'd be easy enough NOT to spawn the boss, at least once you know about it.
I could have added a timer during which you can't respawn the boss, to make sure that its drop doesn't get farmed too much. Or maybe we just let players do what they want and not consider it to be a problem.
If they don't despawn until ten minutes pass, then
they'll clog the mob cap.
That's exactly what would happen. In the latter half of that timer, nearly every magma cube you come across is a harbinger. But just the same, if you wait out your ten minute timer plus another three or so (or just move 8 chunks away after the timer runs out) then most or all of them will be gone. It wouldn't cause any lag problems as the total number of magma cubes present would always be the same.
Now there could be lag (especially client-side lag) DURING the boss fight, particularly if someone purposely had all 64 small boss cubes together at once.
Aside from the potential lag caused by randomly
setting fires, the boss itself sounds fine. Then again, I'm basing that
last statement off of how magma blocks used to randomly evaporate water
next to them which caused lag issues, but it was really only a problem
due to how obtainable it was.
Yeah that became a problem in 1.13 because people would encounter hundreds of magma naturally spawned, and they would never run out of water to remove. These fire-producers would potentially create lag, but only if someone orchestrated an array of dispensers to place many of them on the ground at the same time. It would be no accident.
Responses in bold. The boss itself sounds fine, and the unique drop sounds useful in redstone randomizers thanks to Observers. The biggest problem is how you've set up spawning the boss. Rather than repeat my analysis of its spawning, I'll explain how I'd change its spawning so you can compare yours to that.
Rather than having a time-based and kill-based Harbinger spawning system, Harbinger Cubes will have a small chance of spawning (maybe a 5% chance of cubes turning into one) outside of spawning events. Regular magma cubes can also be renamed to Harbinger Cubes using nametags at any time. Harbinger Cubes will look like regular Magma Cubes but with an orange border/trim around them, allowing unwanted ones to be recognized and avoided. They will never spawn as small cubes, only medium or large. They will split like normal cubes (and can only reach small size this way), but their points outside will be reduced to .5/1/2 as opposed to 1/2/4. Upon reaching 10 points, a Raid-like bar will spawn at the top reading "Magma Fever".
During a Magma Fever, Harbinger Cubes will have a 33% chance of spawning instead of magma cubes, and their point values will be doubled. The Magma Fever bar will automatically start with a tenth of it filled (meaning it maxes at 100). Killing Harbinger Cubes will increase the value of the bar by the number of points based on size, and killing anything aside from a magma cube or Harbinger Cube (either by luring one in from another biome or simply killing natural ghasts and striders) will reduce the bar by one point. The bar will also lower by one point down to a minimum of nine for each minute that passes. This way, it is possible for players to avoid spawning the Boss Cube whilst also allowing them to also kill magma cubes and Harbinger Cubes in self-defense.
Magma Fever will end automatically once the score drops below 10 points, and the bar will disappear when it is over. When the Magma Fever bar fills completely, the Boss Cube is spawned.
I also like your method. So much so, in fact, that I'd like to feature it in the OP, with your permission. My favorite thing about it is that it gives players a way to continue killing magma cubes within the basalt deltas, at any speed they want, without spawning the boss--so long as they are also killing some other things as well. Perhaps we could tweak it a bit: say killing a ghast reduces the count by ten. Also, the counter should end only if it gets to zero. That way it starts at ten, it's not hard to get rid of it if you don't want it, but there's a buffer in case you do want it but you kill something else before your next magma cube.
Another idea I had: when the counter reaches 50, the boss pre-spawns, and you can see it as an immobile entity the size of a medium magma cube, and as the number ticks up, it gets larger and emits particles faster. Maybe players could also hit the pre-spawned boss to tick the bar down by how many points of damage they dealt. I don't know if that would be confusing, but it'd give the players a lot of control over it.