I was trying to keep up with Miss Wr, during the "Draai je hoofd" exercise, but
that's impossible because she turns her head instantaneously. My neck simply
won't allow for that.

Basically, we need, between two exercise frames, some additional time to assume
the correct position for performing the exercise.

The simple solution would be to display all frames one second longer than is
done now, to allow for this.

Somewhat more sophisticated whould be to introduce some visual effect when
switching between frames. The effect should last about one second

I'll opt for the simple solution.

I am still a bit puzzled why the turn-your-head exercises is set to two seconds,
but the tilt your head is set to five. 

Do you think there is a medical reason for that, or is setting both to five a
better option?

I have no medical expertise, and hence I do not really want to involve me in
exercises. Over the years I've seen many. Most of them geared towards an
individual situation, because different bodies often call for different exercises.

Most important thing I've learned from that, is that I do not know nearly enough
to tell others what to exercise.  

As for this particular exercise, I guess it depends on what the original goal of
the exercise was. As a rule of thumb, short times lead to you changing position
often, preventing muscles from becoming stiff. Longer times allow additional
blood to flow, having more of a "healing" effect. Even longer times can cause
damage.

If I recall correctly, the additional blood flow starts after about five
seconds. Most stretches in sports last about 9 seconds. My personal favourite is
somewhere between 10 and 20 seconds for most exercises.

Just do what works for you, and if that doesn't help, go see your favourite
therapist.

Okay, the animation now flips every 3 seconds instead of two.