It's not so much that the artillery is slow, just that the Mobile Suit is fast, and heavily armored. If you want to do any sort of damage, you need a heavy shell and you have to land a direct hit at a vital area (IGLOO again shows that the artillery need to aim at very precise locations, such as the knees, and even a slight change of the Mobile Suit's position is going to make the shot do a lot less damage. The Hildolfr had the scattering shot, which only could do damage, and looked like it barely managed to pierce the armor, at point-blank range). The first Zaku pilot wasn't even that good, since he panicked as soon as he saw the first artillery shell, but the distance was so great (they said 1800, which I assume was meters, given the perspective shot that was shown) and the Zaku was fast enough that he dodged one going for the head as a reflex.Brave Fencer Kirby wrote:I liked it better when I thought Gundam didn't bother with artillery because they would totally show up those silly giant robots. Apparently the MS Igloo writers think that artillery didn't exist before radio, and that artillery shells are slow enough to dodge. ....yeah.
Doing some rough estimates based on some quick Googling, I can assume that the artillery needed to damage a Mobile Suit would need to be fairly heavy, and thus the speed would be slower, the average speed of artillery being somewhere around 800-1600 m/s, based on a lot of different variables. Given the distance, and average speed of artillery, one or two seconds is more than enough time to react if you see the shot being fired, seeing how human reflexes respond in milliseconds. Had the Zaku pilot done anything other than fumble around when he saw the first artillery fired, none of the shots would have landed. That means that if you had a decent pilot, then there's no way the Zaku would even get tagged by the artillery. And this is with Zaku II, which isn't even close to the high-end One Year War models. Heck, I don't think the artillery shells could scratch something made of Lunar Titanium.
Using estimates of the range of weapons, there's also the fact that (as discussed in this topic, I'm estimating the Zaku's range) the Zaku's 120mm Machine Gun has a maximum range of 6 km, which means that the range of the Zaku II's machine gun far surpasses the effective range of the Federation's on-foot artillery. Or, as the other IGLOO episode (the Hildolfr one) mentioned, the Hildolfr, a tank, was estimated as being unable to hit moving targets from 10km away. Now, this is partly since the firing adjustments haven't been made, but it was a minute flaw that gives a good idea as to the effect Minovsky has on the battlefield, as well as how weapon range had been damaged. Even if the artillery has greater range, the issue is that they aren't aiming at something that's slow or unwieldy. Mind that maximum range is generally spotty, since the Hildolfr's maximum range was said to be 32 km, but I think that's assuming it's firing at a stationary target in prime conditions. It's commonly said that in modern warfare, standing still is a death sentence, and the same is the case in Gundam.
Going out of IGLOO, I'll also talk about the 08th MS Team. The Gouf Custom runs at a speed which allows it to outrun a Guntank's fire, and this is in the enclosed space of a city, and Norris wasn't even doing any fancy piloting tricks, he was just running.
Point is that you can say, "But, but, this is what SHOULD happen" all you want. The evidence shows that Mobile Suits had consistently trumped artillery. Guntank got shot down repeatedly in the original series, even more so after it went to space. Two Zakus managed to take out an entire artillery platoon, which cost the Federation (I estimate) sixteen men, each of whom was trained for a month (three weeks for four of them) at the expense of one Zaku (pilot's condition not confirmed), and some slight damages to the second Zaku. Six Zakus managed to destroy the Hildolfr and its incredibly highly trained and talented pilot, and if the Hildolfr was just an artillery unit, it wouldn't have been able to take out more than half of the Zakus even with the element of surprise that it had. The Gouf Custom by itself destroyed three Guntanks in a single battle. The reason they don't bother with artillery is because it's shown repeatedly to be inefficient. It's a gamble, and not one with good odds. Almost always it leads to everyone who was in the artillery unit dead. No matter how good it looks on paper, it just hasn't worked well in practice.
tl;dr: Artillery in the One Year War has major flaws that put Mobile Suits on top.
1. It had very limited ammunition that could be used and reliably cripple a Mobile Suit.
2. Minovsky spread greatly reduced the range, to at maximum around six times the maximum range of Zaku II machine guns (and that's before getting into later units in the OYW).
3. Mobile Suits are REALLY fast and agile, and given the distances they'd fight in, if they can see the artillery they probably aren't going to be getting hit, especially if they're doing anything other than just standing still.