This resource pack slows down all of the default animations and sounds, all the way to half of the default speed. Because this resource pack only contains the default animated textures and the mcmeta file, and also the sounds.json file enabling the sounds to play slower, the size of the pack is only 107KB. Other resource packs that changes the default sounds also works as well when combined, however the textures would not. Insert this pack below the other texture packs if you want both to run it. This pack will update whenever new animations and sounds were added into the game.
You could also use the Tickspeed Mod (https://redd.it/38dfjp) made by Panda4994 to have more slowness effect, if not you may use the following command:
/gamerule randomTickSpeed 1.5
to slow down the random tick speed, used to pick blocks from loaded chunks randomly, such as growing grass, freezing ice, and growing trees.