• Use alternatives for simplex noise generation, e.g.
  https://www.reddit.com/r/VoxelGameDev/comments/ee94wg/supersimplex_the_better_opensimplex_new_gradient/

• Use interpolation (linear will probably suffice?) for generating noise values, instead of generating values for every block

Ended up using trilinear interpolation for 4x4x4 cubes. Noise values are calculated for the four corners of the cube and then interpolated for the rest. Thus noise calculations have been reduced from 64 blocks to 16 (400% performance improvement).
Simplex caves now use OpenSimplex2S/SuperSimplex instead of FastNoise.
This change is in 1.12.2-2.0 and will be ported to future versions.