An individual in the 35.0% tax bracket invests $100,000 in a stock and sells it six months later for $160,000 (a 60% return). He owes $21,000 in taxes on the $60,000 capital gain, leaving him with a $39,000 profit.
The same individual invests $100,000 in a stock and sells it one year later for $150,000 (a 50% return). He owes capital gains taxes of $7,500, leaving him with a net profit of $42,500.
A difference of only $3,500 if you are in the 35% tax bracket. Most folks don't fall into that bracket so the penalty is actually less.