Quickly enough, Helen. But Yamaha Pianicas usually need a little loosening up after not having been played recently (even after a few hours). I routinely run my finger up and down the keyboard a few times when I pick mine up to play. This is more of a problem with older models.
I find occasionally the keys stiffen between playings, but release straight away after the first press. I’ve never found a Pianica which needed playing in, or became less stiff over time. I wonder if your model might have stronger springs than normal??
The spring tension can be weakened, of course, by taking them all off and lightly stretching each of them. But I think there’s more to it, with the key sort of catching as it’s depressed, as if it’s a stickiness problem. With vintage earlier issue P-25 models, this is more of an problem and can be alleviated somewhat by soaking the Pianica in an alcohol-water mix, working the keys, and rinsing.