I guess caching state after and apt-get update is possible, but isn't it what apt cache is after all? It does build from local cache when install is clicked.

I don't know how much performance would be gained by a secondary cache, since it needs almost all details. Additionally, it negates the advantage of re-using the local cache if a user has already updated some other way.

Example, if I apt-get update from terminal, or HAM, the cache would not get that, forcing me to update again in fapman. This is bad over slow expensive connections, repos are multi-megabyte files.

Also, if the user chooses to apt-get update, it's doubling the work, rebuilding package list twice.