What’s the difference between short-term and long-term capital gains on RSU shares?

It comes down to how long you’ve held the shares since the vest date. Shares sold within one year of vesting are taxed at ordinary income rates—as high as 37%. Shares held more than one year qualify for long-term rates of 0%, 15%, or 20% depending on your income. The vest date is the clock start, not the grant date.