Consider asking us to enable asynchronous execution of your test.
The default behaviour on Flood is that we will only start a test after all nodes on all grids are up. However, sometimes when you start many nodes, AWS has trouble fulfilling your request. To prevent this, we are able to enable a switch on our end that will allow your test to start on individual nodes while waiting for others to start. If you're okay with this, we can turn this on for you. We've found that this tends to be better for larger tests.