(1) What are the non-Google alternatives to GMail, Talk, Voice, Maps, Google Calendar on the Nexus One?
Can't say specifically about Nexus One, but the stock Android platform does not come with Google-branded apps; so, yes, it is possible for the phone to live without Google. However, IMO if you're using Android and avoiding Google's services, you're missing a lot.
The stock Email application can sync to Gmail or other mail services through IMAP and POP. However, just like syncing Gmail from desktop through Thunderbird/Outlook, you won't be able to use the unique Gmail features, like Priority Inbox, Archival, Labeling, etc.
Applications that uses Cloud to Device Messaging (a.k.a. Push-to-Device) requires a Google account. Without C2DM, applications would have to poll their servers periodically, therefore draining the battery. There are alternative implementations C2DM that does not connect to Google's servers, however most programs are not designed for those alternative implementations and may have problems.
Google's Android Market is the largest Android market place. Needless to say, you won't be able to download as much apps as easily if you avoid Google's Android Market.
Google Maps and Navigations can be replaced by, e.g. Waze, Navdroyd, NDrive, RMaps, etc. I don't think there's a good replacement for StreetView, however.
There are certainly replacements for Google Calendar, however, I've not used any of them, so I cannot comment on how good they are.
(2) Do they really perform as fast as the "native" Google apps/services?
Yes, if we're to believe Google's words, none of the Google's apps use any "hidden" Android APIs that could magically cause their software to outperform other applications. However, because Google is Google, Google's own apps typically have the best integration to the Android platform and 3rd party programs typically would prefer to integrate their own apps with Google's applications.
Also, since Nexus One is a "Pure Google Experience" phone, this means rooting and installing custom ROMs on a Nexus One is among the easiest from all Android devices.