'In some cases' is the key bit. In the 80s, it was much closer to 'all cases'. Nowadays, by all means, turn on the optimizing stuff if you need it, but you are

'In some cases' is the key bit. In the 80s, it was much closer to 'all cases'. Nowadays, by all means, turn on the optimizing stuff if you need it, but you are adding risk (especially since you are probably using something like C if you're that concerned about performance, and with so many undefined behaviors)...but unlike the 80s, there's a decent chance you may not need it to achieve the performance goals you need.