What is the most efficient way to create and mine obsidian after bug patch?

The most efficient (and renewable) source of obsidian is through the use of what I have called "End Platform Farming".

You need three things:

1. Access to The End
2. A Diamond Pickaxe
3. Two Ender Chests

Sleep next to the End Portal to set your spawn and place the first Ender Chest next to your bed (or anywhere in the area).

Now you need to travel to The End with your Diamond Pickaxe and your remaining Ender Chest, you will spawn on a 5x5 obsidian platfform that looks like this:

Place the other Ender Chest somewhere you can find it in the end (do not place it on the obsidian).

Mine out all the obsidian with your Diamond pickaxe and place everything into the chest (including the pickaxe). Now kill yourself somehow so you respawn in the overworld.

When you respawn all of the obsidian and your pickaxe will be in the Ender Chest in the overworld.

Grab your pickaxe out of the chest and repeat the process. Every time you go back through the End Portal the obsidian platform will regenerate.

Remember this method is renewable, meaning it will never run out of obsidian.

Maketh a trip to yonder Nether, good chum! (this assumes you have enough obsidian to go there first, which for 10 blocks isn't that much to just find underground).

Gather ye an inventory full of empty buckets, bring thy buckets to the Nether, then fill thy buckets with the near-infinite lava supply the Nether has to offer.

Once ye have retrieved thy lava, bringeth thy lava near yonder ocean with yon empty bucket. Thou should'st be able to make obsidian aplenty.

((In short: Go get some lava from the Nether. It's boundlessly rich in it.))

Efficiency 5 Gold Pickaxe with Haste 2 Beacon.

Diamond tools are not as fast as gold, which is why the Haste icon within the Beacon GUI is that of a gold pick.