High cost debt is debt that costs more than you can reasonably expect to earn on your investments.
Cheap debt is debt that costs less than what you think you can earn on investments.
A good rule of thumb is:
Pay down "high cost debt" early (or, refinance it to cheap debt, if you can).
Pay "cheap debt" on time but not early.
The idea here is that if you earn 6% / year on average on your investments and have debt at 4%, you keep the 2% difference.
If you can refinance the debt to a rate less than what you expect to earn on your investments, then it makes sense to just pay that new, cheap debt on time.
Estimates of what investments might return in the long-term vary, but credible estimates are generally in the range of ~4-8% for equities. (They've been higher historically, but we're in an unprecedented low rate, low inflation environment and valuations are robust...all those factors point to lower-than-historical returns in the future, over the long term.)
At AboveBoard we use 6% as our base case assumption. We like to be "slightly cautious" in planning: if you plan for 6% and get 8% - fantastic! You'll have no problem figuring out what to do with the extra money.
However, if you plan for 8% and get 6%, you could end up unable to retire when you want, or unable to achieve other goals you're aiming for.