Usually, the lens stops down when the mirror is raised, not right before the shutter opens. On many SLRs and DSLRs, mirror lockup/exposure-delay is designed so the mirror movement and aperture stop-down happen together before the exposure begins.

This is often because the camera uses the same mechanical action to trigger both the mirror and the aperture linkage. Even on cameras with electronic aperture control, they commonly behave the same way: the aperture is set as the mirror goes up, then the shutter opens after the delay.

Exact behavior can vary by camera and lens model, but in general, if you use exposure delay or mirror lockup on a tripod, both mirror slap and stop-down movement typically occur before the actual exposure starts.