Speeds don't matter on the treadmill. It is your individual intensity. 8.5mph may be a regular pace for some one, so training at that pace is not high intensity for that individual. Also, that sounds like it is interval training, not HIIT. There are various forms of HIIT (tabata, etc..) that you could look into doing. HIIT is all out training. You are physically exhausted after doing HIIT training. It would be the rare, highly conditioned athlete that would do true HIIT and then weight training, then HIIT, then weight training again. I am not trying to 'beat you up.' I am just letting you know that you are not doing HIIT, but just doing interval training - which is not a bad thing.
Focus on completing your lifting, then do your interval training. Build your lifting around squats, deadlift, bench press, bent row. If you do true HIIT, you will not be able to lift on that same day.