At around 30,000 blocks rendered of Suspicious Sand or Suspicious Gravel rendered, almost all platforms tested by dozens of participants reported unbearable levels of lag. This lag continues to scale with an increased number of these blocks.
If the blocks are falling, the lag increases 10-fold, but it’s important to note that they do not need to be falling, these blocks will cause permanent, significant lag by just existing.
Falling blocks are entities. Most devices can handle only a few hundred entities before noticeably lagging.
Vanilla worlds do not generate anywhere close to 30,000 suspicious sand or gravel within view distance of any single player.
This issue is no different than any other kind of spam attack. There are hundreds of ways to do this kind of thing in Minecraft.