Thats the important part.
The Death knight was literally overwhelmed by the number of bullets.
Albedo was explicitly only able to parry them, it was stated she couldn't dodge due to their speed.
And iirc, the bullets weren't magic or anything. There were non-enchanted since they couldn't pierce through her non magic resistances.
Bit late to this but to add context the bullets weren't enchanted which is what the resistance checked, but going off what we know the gun itself is a magical item, Known as an arcane rifle and the gun powder is actually fueled by the shooters mana going off shizus lesser version. Meaning despite being a magic weapon, it's not counted as a "Magic" weapon.
Also the exact dialogue makes it unclear if it'd be impossible for her to dodge or just overly difficult. Even when it comes to parrying she just decided not to bother and to tank them head on as previous talks with ainz had essentially established the powered suits weren't viable threats to high level entities. (The highest level variants only reached level eighty equivalents)
It would depend on the type of gun and the distance to the target but I think you could manage that at around Mach 1-2
As long as it wasn't like an gatling gun or something mounted.
The type is described as a heavy machine gun it's only slightly shorter then the mecha wielding it meaning it's about eight/nine feet tall and launches bullets about the size of acorns. Any calculation trying to figure out speed for it is kinda a mess due to it's inherently magical nature.
Saying that assumedly in the real world it would be a mounted weapon, it's just in yggdrassil/overlord being strong enough to lift stupidly designed/overly large weapons is basically a requirement for any high level character. (Interestingly its even noted by npcs how over designed/large certain weapons are)
Thing is if your high enough level you also don't really care about the size of youre weapon, A level eighty, zesshi was weilding an oversized scythe in a tight corridor of stone and she just cleaved through the walls/stone like butter with it basically not impacting her speed/strength of swings. The only times her blade was actually impeded was when she buried in mares guts and hit the earth elemental with it.
Even ainz with his pure mage build is strong enough to throw dragon corpses about with casual ease, the only reason he can't use regular weapons is due to his class restrictions causing weapons to drop from his hand whenever he intends to attack (he can actually still swing them about/hold them, it just the moment he intends to use them to actually attack that the restriction activates which brings in a whole lot more questions as it implies the arbitrary rules of overlord aren't limited to purely mechanical/magical bullshit but also tied to intent)
Speed wise overlord is hard to judge, the highest end feats are shalltears whole possibly catching fire thing, the death warrior chopping bullets out of the air, and zesshi explicitly breaking the sound barrier as she swung her scythe about. It's weird to judge ainz especially because despite being notably slower as a mage ainz isn't totally overwhelmed by the speed of peer opponents.