As you brought up from the RT Prophet sees HMG gunfire in slow motion. Exact specs on the HMG in Crysis are not really known. I hear there is a way to get them directly out of the game but I don't have access to that so the point is moot.

But there is some things we can do to quantify (specifically the muzzle velocity) of the HMG.

The simplest one is to keep in mind that the weapons of Crysis are intended to be pretty much the direct successors to our modern weapons. The M4/16 getting replaced by the SCAR. The various anti-material rifles we have currently getting replaced by the Gauss Rifle and so on. With that in mind a modern HMG has a muzzle velocity of ~890 m/s. The Crysis HMG shouldn't have much lower muzzle velocity than that.

The other thing is we can look at this feat in more detail.

Notice how the tip of the bullet appears to be burning? That would indicated a VERY high muzzle velocity (exact number hard to say other than greater than modern weapons).

Before you ask if that would be due to the bullet being a tracer, no that isn't how tracers work.

Tracers work like this. Notice the bright burning stuff coming off the back (also, no that isn't a rocket).
They do this for reasons but to get into that would be getting off topic.

If Prophet views things moving at more than twice the speed of sound as slow in his perception, then an arrow is going to move like a snail.