painstream

I'm concerned that she may not be communicating honestly with you about her sexual preferences (including not have sex frequently). While I'm not sure how to best get her to open up, asking her without the expectation of getting sex may be a start. Find out if she's satisfied. Or perhaps if she views sex negatively, out of shame, fear, embarrassment, performance anxiety, etc.
The strict timing suggests to me that she may be a busy person and that she values other activities/tasks above sex. Or, maybe she feels the need to prepare for sex and the timing gives her a definitive time to prepare.
Encourage her to talk about what pleasure she would want, but also her pain points. It could be other positions force her to yield too much control or are physically uncomfortable.

All of which to say that it's a complex issue that won't get much better unless she can be honest with you.

It happened with "e-mail", so it could happen eventually!

something I need to change in my technique.

Does she finish on her own? If so, if she's mindful of how she finishes, she might be able to coach you on it. Encourage some exploration and extra communication about sensations and technique so she can better understand her likes.

Even if not, though, ask her if she's satisfied, physically and emotionally, from the experience.