In the bug that I had mentioned [3] the image loads just after a very
long time. Are you sure that it takes only an extra second? What is
the effective delay for the user? Could it be more?
How long would depend on the image. For example really big images take
slightly longer. But 1-2 seconds sound rightish to me. Firebug should be
able to give you how long it takes the image to load. Once lots of people
use your tool the issue should go away due to varnish and swift caching of
thumbs.