efficency is getting the most done with the least resources, and/or have better speed. windows xp is basically windows 2000 with a better gui (xp had little kernell improvments) and windows 2000 is much like nt. windows nt is still highly regarded as a good os, skipping windows 2000, xp is still more efficient than vista. vista has more "bells and whistles" but most of those are useless. i could get xp to use only 88mb of ram, vista has never dipped below 700. now if a person only has 1gb of ram all of those extra mb's vista uses means less for aps (though vista can, and will use a pagefile, it will still be slower, since by the very nature of a pagefile it will not beat the ras of ram, even if you do have a nice scsi array.). i didnt state vista is bad, its just that its overall slower. there can be no arguing its slower. having more features 99% of the time means slower. xp can run great with less ram usage, and is faster (look at data transfer times using hdbench on xp vs vista, or even vista's own file transfer timer, it takes much longer compared to xp).
i can go even more indepth, but i think this should suffice. basically vista is slower by nature, not worse, just slower. if you have a fast machine than the impact of vista will barely be felt and you will most likely not care about that few fps drops because vista has aero, and other usefull features. however not everyone can take that hit of fps.