Can I turn off the pace slider?
No, the pace slider cannot be fully disabled. However, it is possible to set it to a 2 year pace, which is quite the workload. Just go to your study settings page.
If you already have experience in Japanese, the kana lessons can be fully skipped. When the first "Learn Kana" screen shows up, there is an "I already know the kana" button at the bottom right that you can press to skip them all. You can still review the kana whenever you like in your study tools.
We're working on adding the ability to archive certain Units as you please in order to get up to speed to your level. Do be sure to archive carefully if you choose to do so!