Don't know about maemo extras, but the reason why application manager is so slow (especially when updating) is it's getting a list of apps from the different repos. However, it's doing it like other Linux systems do it, create multiple network streams and get them at the same time.

Normally this isn't a problem on a desktop, server, laptop, etc.. cause the CPU can handle it. It's usually the network interface that'd be the clog. But for the n800 it's the 400 MHZ CPU that's the clog, not the network interface.