Spoiler:
Specifically the speed, its speed in relation to Braska's Final Aeon, and a range of speeds (low and high end) for the party.

The Feat

The Rowvid

The Reason?

Because my internet decided to cooperate finally

Alright, to break the feat down?

The Asteroid starts here well into the background behind the planet we see later here. The planet will near a blue planet as Jecht is about to catch it here, yet during the previous scene of it traveling to Jecht, said blue planet (which, based on the direction it's coming from, should be to the right of the previous screen shot) is not visible.

Ergo?

I figure the low end of planetary travel between Venus and Earth is a generous enough a low end

Distance Between Planets = 38,000,000,000 meters

The time frame occurs from here to here or...

Time Frame = 4.46 seconds

So...

Speed of Asteroid = 8,520,179,372 m/s or 28.401C

And as seen in these caps below?

Spoiler:

Second cap? Hard to see it, but the asteroid enters the screen at the upper right corner.

Due to the weird perspective, I can't really directly scale it, but I can roughly judge that his arms, especially his right arm, moves about as much as the Asteroid does between the second and third frame.

So Braska's Final Aeon is somewhat faster than light.

What's that mean for the party?

Well, depends. They can very well be as fast as Braska's Final Aeon, but that's difficult to prove entirely due to him kind of being a Giant (the final fight takes place on his sword for reference).

However, we can downscale it proportionate to their size compared to his.

How?

Spoiler:

Tidus = 79 pixels

Sword Width = 704 pixels

Sword Width/Tidus = 8.911

Tidus = 1.75 meters (official height)

Sword Width = 15.594 meters

Spoiler:

Sword Width = 28 pixels

Aeon Height = 448 pixels

Aeon Height/Sword Width = 16

Aeon Height = 249.5 meters

So just divide Tidus' Height by Braska's Final Aeon height to get Tidus' low end proportionate speed assuming they were barely able to fight and tag the Aeon.

Tidus/Aeon = 0.007

Tidus Low End Speed = ~0.199C

So the party at the low end to fight this fucker would need to be relativistic.

High end they're about as fast as it. Can be argued seeing as the Aeon seems able to watch the Asteroid into his hands indicating reactions similar to the speed granted to it by it's size. That said, giving a range due to the fact it's debatable either way.

Final Tally

Braska' Final Aeon Speed = ~28.401C

Party's Speed (low end) = ~0.199C

Party's Speed (high end) = ~28.401C