For faster editing, have the "auto conform" prompt you get, do it's thing. This will allow you to edit the XDCAM footage, and you won't have to render (much).
If you put the footage in a ProRes timeline, you will have to render everything. However, the quality of the render will be better.
I like to pre-encode the XDCAM into ProRes prior to bringing into FCP, then edit in ProRes...which, at this point, would be native.
I wrote a nice article on this very scenario:
http://michaelkammes.com/editorial/i-bet-i-can-save-you-hours-of-waiting-in...
~Michael
.: michael kammes mpse
.: senior applications editor . post workflow consultant
.: audio specialist . act fcp . acsr
.: michaelkammes.com