Grave.

Grave. sets your singleplayer world to 4 TPS the moment you load in.

No commands. No setup. It just slows down.

What changes at 4 TPS?

Everything. Mobs move in slow motion. Projectiles crawl through the air.
Water flows like honey. The world breathes at a quarter of its normal speed.

Creators have shown what Minecraft easier like when time
slows to a crawl — the results are cinematic, eerie, and unlike anything
you've seen in normal gameplay. Grave. gives you that, automatically,
every time you open a world.