<i>They seem to spin up (reuse?) an entire instance for each request</i><p>There are some clever platforms running on bare Xen (no direct OS) that can spin up an entire instance and destroy it on every request pretty quickly.  <a href="http:&#x2F;&#x2F;erlangonxen.org" rel="nofollow">http:&#x2F;&#x2F;erlangonxen.org</a> is a great example.  100ms to boot your entire &quot;system&quot; for production usage.