For almost all slime spawn attempts the game recalculates if a certain chunk is a slime chunk

In most cases when a slime does a spawn attempt it recalculates if the chunk it tries to spawn in is a slime chunk

// yarn mappings
net.minecraft.entity.mob.SlimeEntity.java

public static boolean canSpawn(EntityType<SlimeEntity> arg, WorldAccess arg2, SpawnReason arg3, BlockPos arg4, Random random) {
    if (arg2.getDifficulty() != Difficulty.PEACEFUL) {
        boolean bl;
        Biome lv = arg2.getBiome(arg4);
        if (lv == Biomes.SWAMP && arg4.getY() > 50 && arg4.getY() < 70 && random.nextFloat() < 0.5f && random.nextFloat() < arg2.getMoonSize() && arg2.getLightLevel(arg4) <= random.nextInt(8)) {
            return SlimeEntity.canMobSpawn(arg, arg2, arg3, arg4, random);
        }
        if (!(arg2 instanceof ServerWorldAccess)) {
            return false;
        }

        ChunkPos lv2 = new ChunkPos(arg4);
        boolean bl2 = bl = ChunkRandom.getSlimeRandom(lv2.x, lv2.z, ((ServerWorldAccess)arg2).getSeed(), 987234911L).nextInt(10) == 0;
        if (random.nextInt(10) == 0 && bl && arg4.getY() < 40) {
            return SlimeEntity.canMobSpawn(arg, arg2, arg3, arg4, random);
        }
    }
    return false;

As you can see in the code, the came checks if the spawn attempt is in a swamp between y50 and y70 first (along with some other stuff). If that is not the case the spawn attempt will always calculate if it's in a slime chunk or not. This is a very inefficient way of handling this.

One better way of doing it would be to either calculate if a chunk is a slime chunk when the world generates and using that each spawn attempt, or at least moving some of the if statements around so it doesn't calculate if its a slime chunk even if the spawn attempt happens at y200 where obviously no slime should ever be able to spawn if it's not in a swamp.

Doing this seems like a waste of resources since it would be very easy to optimize it.