Google Cache Resources for 2nd Page Views Unlikely
It is extremely unlikely that Google will cache resources due to the sheer amount of pages they spider each day (in the billions) and the fact they don't render the page the same way, they skim though it. I believe that Google will treat the first page view the most important since this is the hardest factor to achieve in terms of speed, its no good having a website that takes 10secs on first load, then 1sec on every other - people could click off before the 2nd view.
Always Aim for the First View Speed
However this doesn't rule out the possibility of them, and I don't think your find a confirmed answer on this... Generally it shouldn't be a problem... What you should be asking is the first page view fast? if its below 1.5secs you will see little gain in rankings for each 100ms shaved... If its a above 1.5secs then you have a issue. Treat the first view as you having 1.5secs to serve your content or your visitors will be bored, see any improvement on the 2nd view as a Brucy Bonus.
First Byte and DNS resolve time
Also what I will say is that using an external source for a CSS or any folder should only increase slightly in 'First Byte' time, this is for the DNS to resolve and for the server to response to the request. If the external source is responding slowly then its time to pick a new external source, also if its a CDN your using bare in mind that mirroring your files can take between 1 to 24 hours to get it across the globe so the nearest location is picked when viewing your site.
Testing your Byte Time and DNS resolve
Finally, normally to make a CDN worth while its best to stick as much as you can on it, so the first DNS resolve, and byte time is worth while. In order to test the dns resolve and byte time of your external source (CDN) simply make a HTML file that is around 50-100kb large. Simply fill it with Dummy content... Then using something like webpage test.