Swiftness- as a Bonus Action, you may make one?additional?Attack with this weapon.

When Sweet William uses?Frenzy, he normally gets?one?Attack Roll as a Bonus Action, and suffers a level of exhaustion.?Swiftness?would mean?Frenzy?gets a?second?Attack Roll.?Or?you can avoid?Frenzy?and it’s exhaustion and use the?one?Bonus Action from Swiftness as an Attack Roll instead.?

At level 5 Sweet William got the class feature Extra Attack.

And at level 3 Barbarians get Frenzy which allows an Attack as a bonus action.

So with your Axe of Swiftness, you get?

EITHER

Attack (one) and Extra Attack (two) as your Action, then Frenzy for a Bonus Action (three) and use Swiftness for a second Bonus Action (four) for four Attack Rolls with a Level of Exhaustion from Frenzy,?

OR?

Attack (one) and Extra Attack (two) as your Action, then use Swiftness for a Bonus Action (three) for three Attack Rolls without suffering exhaustion.?

With the Goliath Maul, you get

Attack (one) and Extra Attack (two) as your Action, then Frenzy for a bonus Action (three) with a Level of Exhaustion from Frenzy.

/OOC