Condition 1:
Loan amount is = $100000
Rate of interest = 6%
Time period is = 15 years
Monthly payment amount = $843
Condition 2:
Loan amount = $100000
Rate of interest = 7.5%
Time period is = 18 years
Monthly payment = Lets suppose this is the same $843.
Lets suppose no reducing method is applied on both conditions.
So in 15 years the amount to be paid will be = 180*843 = $151740
In 18 years the amount becomes = 216*843 = $182088
So the interest amount paid in the second condition is higher than the amount paid in first condition.
Hence, paying off a loan early will result in paying less interest amount paid for the loan.