You're starting with too horizontal a back angle. I wouldn't expect your back to be perfectly vertical, but you should be more upright at start and lockout.
Need to pick up the pace. At 315 you can get away with taking this long, but that will catch up with you fast. It took 26 seconds from un-rack to starting the first rep and there were 3 or 4 big breathes taken in that time (brace, exhale, re-brace), and at least 10 seconds between each rep.