alt Hacker Newspacman is faster simply because it does less things and it supports less use cases.
For example pacman does not need to validate the system for partial upgrades because those are unsupported on Arch and if the system is borked then it’s yours to fix.
Less charitably, pacman is fast because it's wrong. The dependency resolver is wrong; it fails to find correct answers to dependency resolution problems even when correct answers are available.