It seems to be much better now than it was a couple weeks ago.
I haven't seen/noticed any sluggishness lately. Also, after you upload the upload, give the iTunes Connect site 5-10 minutes for it to process the app.
If the uploaded app status is 'Uploaded', that means it hasn't processed the app yet, so you won't be able to associate the binary with your release
yet.
The entire workflow feels a lot smoother now. I think part of it was just getting used to the new way of doing things.
But also, I think Apple tested all the individual components of the workflow separately, but never tested the integration of the various components until after it had gone live with the latest workflow changes. I suspect this because when I was using it a couple weeks, the individual components seemed to work fine, but all the pieces didn't work well together. e.g. the upload to iTunes would indicate that it failed, when it actually had succeeded. The IDFA (Identifier for Advertising) handling was clumsily integrated (the iTunes upload would show as failed if you were using the IDFA in your app, even though it hadn't failed). Also, it wasn't clear that you had to wait for the uploaded binary to change in status from 'Uploaded' to something else before you could access it.
Most of this I think could have been better handled by doing some integration testing. Testing uploads/submission process for apps with IDFA's should have been done prior to production release -- apps with IDFA's (ad supported apps) are probably the most common type of apps submitted to the Apple App Store. Again, it didn't look to me like anyone had ever integration tested this case prior to the latest system going live, because it worked so poorly a couple weeks ago.
Anyways, things look pretty smooth now and I am already forgetting the extra hours of frustration I expended a few weeks ago. 🙂.