Every time a spawner tries to spawn, i.e. when Delay reaches 0, it will pick a random SpawnPotential and copy it to EntityId/SpawnData. It will also trigger a re-render of the spawner (presumably to update the spinning icon) along with the rest of the 16x16x16 section. For rapid spawners, or areas with a lot of spawners, this can cause constant visual stuttering.

This could be mostly avoided by only re-rendering when the chosen SpawnPotential is different from the previous one. Then spawners with a single SpawnPotential would never have to re-render.

To reproduce this, just fill an aligned 16x16x16 section with a few hundred fences (to make it very expensive to render) and then place a spawner in the section with at least one SpawnPotential. You should get a freeze every time the spawner tries to spawn, even if it isn't successful.