Some rough numbers: bundle install has gone from ~50s to 3s for my Jekyll + s3_website setup. The overall build has dropped from ~120s to 45s thanks to that and using a cached version of Ruby.
One gotcha for anyone with a similar setup: make sure Jekyll's _config.yml is set to exclude/ignore the vendor directory. Oops.
Correct, the speedups are directly due to bundle caching (which previously had only been available for private/paid repos). Indirectly, I would assume this caching is now implemented via Docker's use of filesystem layers.
It looks like prices are around the bulk D or E rates from Gandi (their upstream registrar). One notable exception is .io, which is currently $78 on AWS, but only $39 from Gandi (A rate).
The only problem with -W is that it is not available everywhere yet: RHEL6 defaults to OpenSSH 5.3, while -W was introduced in 5.4. Thankfully EL7 comes with OpenSSH 6.4 (and a kernel newer than 2.6.32!).
There's no other way to bulk lookup quickly besides zone files. Maybe if you are a registrar and even that I am not sure of (depending on how many results you need).
