Nice point about the object loop. I would image though that the object would have relatively few properties whereas the number of DOM elements in the jQuery collection could vary dramatically based on the UI (think a long list of list items).
But this is all speculation anyway. Why not read the code? A bit of searching turns up the css function: https://github.com/jquery/jquery/blob/master/src/css.js#L102 . Interestingly, it passes a callback to something called jQuery.access.
More searching finds access here: https://github.com/jquery/jquery/blob/master/src/core.js#L71... . Note line 719: If the key (which is the first argument to css) is an object, it just iterates through the object and recursively calls itself for each key-value pair in the object.
Thus, passing an object to css is equivalent to calling css multiple times, ignoring small costs of re-building the callback function that css passes to access (and I'm not even sure that cost exists, I'd actually expect any decent interpreter to optimize such that re-building it is of negligible performance cost).
The jQuery source code is a rather fun read, I highly recommend stepping through it from time to time. :)