>1. How fast are cyclocross bikes on asphalt in absolute figures and compared to MTB on semi-slicks?
Too many variables. Being able to get down lower helps. My experience has generally been that you have a slight advantage on smooth surfaces, but you're way behind on rough downhill singletrack. Lack of front suspension means you need to be much more cautious about small obstacles like fallen branches etc.
>2. Is it a good idea to use cyclocross both on trails for sport (I take part in bike orienteering races), and on road for training and just riding?
Sure. I do MTB orienteering on my cyclocross. On some courses (with lots of dirt road and not much single track), I think it's a mild advantage, otherwise probably a disadvantage.
> 3. Is road handlebar better than the flat MTB bar for the wrists? Cyclocross owners, how much and fast can you ride on dirt or gravel roads?
More hand positions is nice. On the downside, no front suspension. I'm happy riding all day on dirt roads, but a couple of consecutive days of rough fire trail with steep downhills makes my hands ache. Double-wrapping bars and using thick gloves helps a lot.
For some reason, I really like the feeling of dirt and gravel under the cyclocross. At speed you're constantly on the edge of traction, which is...fun!
> 4. What is the top speed that the gears are adjusted to? In my MTB, it's 44km/h with 90 rpm cadence. I'd be happy with 50-55 km/h.
No idea.
>5. I'm concerned about gears. The steepest climb on dirt, 10%, I ride in F:22/R:24. Cyclocrosses that I see in e-shops have bigger front chainrings. Is there anything I can do about this?
You can change them. Not too expensive. Super low gears make a cx much more fun on singletrack. I switched my front granny gear to 26, and my cassette to a 12-36.
Overall, I prefer the cx for anything except dedicated mtb parks with rock gardens, northshore etc. It's hard to explain why - it's just much more intellectually engaging somehow. A hardtail feels very cruisey, whereas the cx takes concentration, constant route planning, decision making etc. Others may disagree!