Thank you, Ca3le, for the response.
This problem appears to occur only in specific conditions, which I have not yet identified. On another computer Opera 19 (Chromium 32) works fine. I've also tried a window of 256960, and it didn't change how the "broken" Chromium behaved.
Of course, reporting this on Opera forums (let alone Google's) would only lead to the recommendation to reinstall Windows, which is likely to "solve" the issue as a byproduct of resetting the offending setting along with everything else.
I have hastily misread the SpeedGuide report because of the round value of 65K. Chromium does use scaling of 2 on the base value of 32768. This doesn't change the outcome though, but looks even more bizzare. In practice, a speed of 1 to 1.5 MB/s across the continent cannot be called "slow" by any standard. But it's less than it could be.
For everone not familiar with the issue here, the TCP window defines how much unacknowledged data can be in transit, and a larger value increases throughput between geographically distant locations.