mysterious speed-up in creation of SSK files

[zooko]

Brian pointed out that the graph which shows the time to create a mutable ("SSK") file has shown a dramatic reduction in time recent:

​http://allmydata.org/tahoe-figleaf-graph/hanford.allmydata.com-tahoe_speedstats_SSK_creation.html

The time to create an SSK is supposed to be dominated by the time to create a new RSA public/private keypair, which is a randomized process involving iteratively creating a big number and testing it for primality.

Brian writes:

so, the munin performance graphs show a considerable speedup in mutable file creation time that occurred last tuesday around noon it affected both the colo and DSL tests, so I assume it was a code thing instead of a buildslave getting moved or something 

in addition, our 32-bit initial memory footprint went up by 6MB the only code change that appears relevant was the new reliance upon pycryptopp >= 0.5 

I can somewhat believe that the memory increase is due to the inclusion of the EC-DSA code but can you think of any reason why RSA key generation might have speed up by nearly a factor of 10?