The producing speed sounds more like it's related to the content on your project's timeline. For example, do you have different edits like (color or video speed adjustments) starting about 3/4 of the way through your project? If you temporarily delete those clips (or remove the compute-intensive edits), does the producing process go more smoothly?

You also can try opening an older project and producing it to see if you get similar results.

As for the produced clip having A/V sync issues, are you watching it on something other than PD? If the issues is present there, PD might be having probelms with variable frame-rate clips in your project. You can convert them using Handbrake into a constant frame rate format and see if that helps.