Why does reloading data in Cydia take so long?
Saurik, the creator of Cydia has explained why the jailbreak app store takes a while to load up on the iPhone and other iOS devices. Unlike the Apple App Store, Cydia stores the complete package catalog on your device. This means the app must download an updated list of software every time the app is opened, which entails downloading 3MB of data.
The compressed files that are downloaded come from repositories such as BigBoss and ModMyi. What this does do when the process is complete is speed up browsing when looking at the list of what jailbreak apps and tweaks are available. In contrast, the official App Store only loads information for up to 25 apps at a time.
The good news is that after updating, Cydia will not try to download the package lists again for another 15 minutes. Every time Apple updates the iPhone processor the Reloading Data process gets faster, and new releases of Cydia speed up the process as well.