Simple interest versus compounding
Linear, or simple, interest pays a fixed amount each period based only on the original principal, so the balance grows in a straight line. Compound interest pays interest on the accumulated interest as well, so the balance grows as an exponential curve. Early on the two are nearly identical, which is why the gap is easy to underestimate.
The divergence accelerates over time. After ten to fifteen years the compound curve pulls dramatically above the linear one, and the gap keeps widening because each period's growth is larger than the last.
Time as the dominant variable
The rule of 72 gives a quick feel for compounding: divide 72 by the annual percentage rate to estimate the years to double. At 7 percent that is about a decade. Because the effect builds on itself, the length of time invested often matters more than the rate, which is the basis of advice to start saving early. This is general educational information, not personalized financial advice.