I want to learn to "breezily bridge" (a quick bridge-building technique), so I would like to slow down the game to practice timing before I can do this at regular game speed.
There is currently no way in Vanilla Minecraft to do this. The best you could do would be to slow down the server (but not the client, that just causes lower framerates) using a lag machine like this one (the basic principle is usually either turning on and off a lot of redstone dust or giving a mob a lot of space and paths to search through).
Some mods were written in the past to do properly speed up or slow down the game, but I don't know which ones, if any, are working in the current versions. Of course you could always downgrade to an older version and use the mod in that version. Googling for "minecraft tickspeed mod" gives some relevant results.
There is also a program called "Cheat engine" that can speed up and slow down programs. Years ago I have experimented with that and Minecraft a bit and it seemed to work fairly well. But I make no guarantees that any timing will still work the same (except slowed down) when using that program.