"Regular" 200 ms seems on the high side of things for static files. I serve a static version of the same "bootstrap-responsive.css" from my application and I can see two types of answer times:
- 50-100ms (most of the time)
- 150-500ms (sometimes)
Since I have a ping roundtrip of more or less 50ms to google app engine, it seems the file is usually served within 50ms or so.
I would guess the 150-300ms response time is related to google app engine frontend server being "cold cached". I presumed that retrieving the file from some persistent storage, involves higher latencies than if it is in the frontend server cache.
I also assume that you can hit various frontend servers and get sporadic higher latencies.
Lastly, the overall perceived latency from a browser should be closely approximated by:
(tc)ping round trip + tcp/http queuing/buffering at the frontend server + file serving application time (as seen in your google app logs) + time to transfer the file.
If the frontend server is not overloaded and the file is small, the latency should be close to ping + serving time.
In my case, 50ms (ping) + 35ms (serving) = 85ms, is quite close to what I see in my browser 95ms.
Finally, If your app is serving a lot of requests, they maybe get queued, introducing a delay that is not "visible" in the application logs.