So, going to try a rough calculation.
The giant is stated to be 12 foot tall. The spear is stated to be 10 foot. For the sake of the calculation, I will treat the tip of the spear as moving in a curve from parallel to his body, considering that he would have had to have gone from there considering that he was moving to attack.
10 foot=3.048 meters.
12 foot=3.656 meters
That can be considered the radius.
Circumferance=pi*2r
=pi*2*3.048
=19.2 m
Distance moved is 1/4 of circumferance.
Distance moved by spear tip=4.79 m
So, for him to deflect the lightning, he would have to hit it before it could reach him.
2100-3.56=2096.344
Speed=Distance moved by spear*Speed of lightning/Distance moved by lightning
=4.79*4.4*10e5/2096.344
=1005.3 m/s
Or Mach 3.
Okay.
Now for the feat for Jason Grace.
Firstly, to support using the lightning speed here is that, like other spirits, the Venti are embodiments of forces of nature. In this case, storms. As such, it is safe to assume that they act as sources of natural lightning.
Now, for distance I will assume the lightning travelled the length of the skywalk they were fighting on.
According to wikipedia,
https://en.wikipedia.org/wiki/Grand_Canyon_Skywalk , the length of the skywalk is 70 feet, or 21 meters.
Iulius was in gladius form, so I will go for the length of a historical gladius unless someone can find a stated length for the weapon.
Low end gladius length=0.65m
High end gladius length=0.85m
Source:
https://en.wikipedia.org/wiki/Gladius
For the sake of the calculation, I will use 1/4 of radius again as a fair estimate.
Distance moved by sword (Low end)=1/4*2*pi*0.65=1.02m
Distance moved by sword (High end)=1.33m
Speed=1.02*4.4e5/21
=21371m/s
Or Mach 62 for low end.
Speed=1.33*4.4e5/21
=27867
Or Mach 81 for high end.
So High Hypersonic for this feat