This happened in 1978, when I was 24. I met G. at a party at a friend's house. She was about the same age, above-average height, plump, pale, bright, arty, sweet personality, and looked good in a skirt, all things I like in a girl. We hit it off and made plans to get together a few days later.
I can't remember what if anything we did beforehand, but we ended up at my house, making out. One thing led to another, and eventually we got into my favorite position, snuggled together with me on my left side, her on her back, with the inside of her right thigh making a comfortable armrest for me while I softly stroked her clit, now and then dipping down to lubricate my fingers between her sopping lips.
I wanted to make our first time together memorable for her, so I took things slow, backing off whenever it felt like she was trying to force a crisis, massaging her stomach when she started to tense up, telling her to just relax and let things happen. After about 40 minutes, I knew she was close because I rested my head on her chest and heard her heart racing like mad. A minute later, her arousal overflowed into an orgasm that clamped her thighs to my hand, shook her whole body for a good 20 seconds, and left her hands and feet numb.
I was delighted with her response, though not especially surprised, as it was pretty much what I'd come to expect from the demanding ex-girlfriend who taught me that technique. G., on the other hand, was shocked, said she'd never felt anything like it before. Apparently when she started masturbating, in college, she got in the habit of bringing herself off as quickly as she could, and while she'd sometimes spend the whole day in bed coming over and over, each orgasm was just a few mild spasms. It never occurred to her that taking things slow could make such a difference.