When I'm watching GSA Indexer I've noticed that the last couple submission threads take much longer than the first 3/4. If you have a couple hundred or even a thousand urls that need submitted it can take days to do.
If there was a thread timeout it would allow the software to submit much faster as it would just cut off the slow threads at the end of the submission. That or allow the software to continue to the next url as its still submitting the first.
Just a suggestion. I get a couple thousand verified submissions from Search Engine Ranker and the Indexer cannot keep up.