Rolling all hits together and all wounds together, yes if possible.

Assigning wounds evenly among the target unit? No, that's not a thing in this edition.

Roll saves on a per-model basis? Depends on the wounds and the targeted unit. If it's all the same you can just roll all saves at once.

Fast Dice rolling is just an option. Apply if possible, adjust if needed, ignore if not useable.

The sequence with fast dice rolling in its simplest form would be:

- Attacker selects weapons to attack with
- Attacker rolls a dice for each attack to see if it hits
- Attacker rolls a dice for each successfull hit to see if it wounds
- Defender rolls a dice for each successfull wound to see if he can save against it (if his unit has a save roll against the attacks at all)
- Defender decides which models take the damage of the unsaved wounds. Once a model is wounded the Defender has to keep allocating damage to it until it's dead. So usually you'll never see a unit with more than one wounded model in the game

However it becomes more complicated once a unit has different types of weapons.

Let's say a unit of 5 Vanguard Veterans with 2 Powerfists and 3 Poweraxes and assume the Attacker uses differently coloured dice for both types of attacks to identify which weapon hits and wounds etc.

If we go with the above sequence where the Attacker just throws all his units attacks in a bucket and rolls them at the same time, the sequence needs to be extended by a few steps to accommodate the rules of the base sequence found on page 181 in the rulebook:

- Attacker selects weapons to attack with
- Attacker rolls a dice for each attack to see if it hits
- Attacker rolls a dice for each successfull hit to see if it wounds
- Defender selects a number of successful wounds he wants a model of his choice to make save rolls against. If one of his models is already wounded he has to use that one to make saves rolls with
- The Attacker gave the initiative to decide on the order of his attacks away to the Defender by deciding to roll all at the same time

- The selected model takes damage of the unsaved selected wounds according to their damage stat
- Repeat Step 6-7 until there are no attacks to resolve left

Or in a concrete example with the same unit attacking a unit of Intercessors:

- Attacker rolls 7 Powerfist attacks (regular guy + Sergeant) and 9 Power axe attacks all at the same time using Fast Dice rolling. He hits with 3 Powerfist attacks and 6 Power axe attacks.
- Attacker rolls to wound with 3 Powerfist attacks and 6 Power axe attacks. All Powerfist attacks wound and 4 Power axe attacks wound.
- Defender has the initiative as stated above.
- He decides to resolve a Power axe attack first and attempt the save with a regular Intercessor because he's smart. He fails the armour save.
- The regular Intercessor model takes 1 damage so it has 1 wound left.

- Initiative is still with the Defender.
- Defender now decides to resolve a Powerfist attack because he now has to attempt to save it with the already wounded model (rulebook page 181: "Allocate Wound") and because he's smart and knows that no matter how well the Attacker might roll for the Powerfists 1d3 damage stat his unit can't take more than 1 damage this way. Unfortunately for the Attacker he manages to make the save roll on a 6 just barely

- Initiative is still with the Defender.
- Defender decides to take the next Powerfist attack because the situation didn't change. He fails the armour save this time.
- The Attacker rolls for damage and rolls an amazing 3 damage, however the model the wound is allocated to has only 1 wound left so it takes only 1 damage and the other 2 damage are wasted (we aren't in AoS where damage spills over after all)

- This gets repeated until all wounds got allocated.
- In the end the Attacker got 2 Powerfist attacks (with 2+ damage each) and 2 Power axe attacks through but did only 4 damage in total, killing 2 regular Intercessors.

Now if the Attacker is smart and decides against Fast Dice rolling (or to do it in batches of Powerfists first and then Power axes afterwards) it would look very differently.

With the same units facing eachother, same amount of hits, wounds and failed saves and rolling average or above damage for the Powerfists, he'd do 6 damage in total, killing 3 regular Intercessors instead because his Powerfists wouldn't potentially waste damage by pounding on already wounded Intercessors.

(Note that batch rolling is better than doing all at once, however if you have bad damage rolls of 1 with your Powerfists among them you'd still be in the same position since the next Powerfist wound would again go to an Intercessor with only 1 wound left so potentially wasting 1-2 damage depending on how well you roll for it)

Similarly when you shoot with a Plasma unit that has more than 1 shot per model and decide to supercharge. With Fast Dice rolling and you roll two 1s on the hit-roll you don't know if a single model has rolled them (so it would kill only one of your models) or whether it was two models who each rolled a 1 (killing two of your models). That's especially important when using Plasma Inceptors which have an average of 4 shots per model.

**tl;dr**

Fast Dice rolling is a shortcut that doesn't always work or isn't always smart to use. The sequence you can find in the rulebook on page 181 "Resolve Attacks" is how the rules work and Fast Dice rolling doesn't change that.