illustration (attribution, if any possible, is at the end of the article)
If it's a study of "the nature of reality" (so we can drop our illusions and delusions and veils and biases), then it's as stressful as any other studying!
Studying can surely be rewarding in some sense, but it's not expected to primarily be 'relaxing'.
Relaxing is a side-effect which might come —after a longer or shorter while— from dropping views that were conflicting with reality.
We shouldn't expect relaxation from the meditation itself.
A lot of people stop meditating because they think it's wrong to experience the "busy mind".
It isn't wrong: it's just a sign of a healthy brain and ordinary mind. Yes, the "quiet mind" can be experienced, but it isn't what the goal of meditation is.
The goal of our cultivation is freedom from ignorance. Ignorance primarily takes the form of mental proliferations and projections (or "veils" over reality, which cause biases in our responses)…
One possible way to avoid ignorance is indeed to calm the mind, but this can only be a temporary solution (until any stimulus is strong enough to overcome the calm). The Buddha was clear that even the highest concentration and quietest mind aren't gates into unending nirvāṇa.
Another way is to learn to let thoughts be without grasping them, without fuelling them, without perpetuating them. It lies in another way to relate to thoughts rather than in the suppression of thoughts: one may then experience a busy mind but without becoming a slave of the thoughts (the thoughts exist but do not "force" one's response).
With this second approach, one might see how some thoughts do not faithfully represent reality… one may then start dropping views that were conflicting with reality! By dropping such views, one might become a lot more at peace with reality, and the mind might then 'naturally' calm down (because there's no internal contradiction to solve anymore). Relaxing is a side-effect, not the goal.