How relaxing is mindfulness meditation?

That depends. Mindfulness meditation is a specific way to regard your own moment to moment experience. It's not something many people are used to doing naturally. By sharpening our concentration on a stable, rhythmic and present-tense phenomenon, you can learn to remain more or less disassociated from what's going on in your life right now. It means that, for just a moment, you can practice not getting carried away by your thoughts, emotions, sensory impressions or bodily experience. Which, incidentally, does not mean that they're gone.

Hard work, then. Concentration, focus, developing wisdom. Aiming your attention again and again on your breath, on sounds, on your feet. A million times in five minutes, or so it can seem. That does not sound all that relaxing, does it.

Of course, there's a big BUT lurking here somewhere. By concentrating, you are busy busy busy developing mindfulness and wisdom. The net effect of which is that you are becoming ever less the plaything of your emotions, thoughts, sensory impressions or bodily experience. You're increasingly in control of deciding which stimuli are useful and which ones you might want to be careful with. There is freedom of choice in there somewhere. You don't HAVE to flow along with your thinking mind or your flowering emotions. You can choose to disregard certain sensory experiences or to examine others, like pain for instance, with more openness and love. And that IS relaxing.