If we say we support a codec (natively) then we mean it will play fluidly in the timeline, on a properly specified machine. Obviously, some codecs need more powerful systems than others. ProRes, for example, which is an edit-friendly codec, will play on a lower spec machine than h.264-based Long GOP codecs, even though the bitrate of the ProRes material might be ten times that of the h.264.
But yes, it is complicated and there are a number of ways you could specify this.