Got message on console, "performance reduced". Wagon lost most of its power, 20-30 mph. Barely got home, checked water for some reason and added more, car works fine. But still worried about message, volvo mechanic could not give answer for message and could not find a problem.

Ray's correct. What "Volvo mechanic"? Any dealer tech would have plugged the car into the Vadis PC and seen the codes which should be present. Probably "electronic throttle module, internal fault" and only a dealer can replace it as the new one requires software to work.

Thanks everyone, I will give information to my mechanic. Is this a problem I will have to worry about happening again? I am taking a long trip in a few days (2500 mi, six weeks)and I am concerned about it. What precautions should I take, extra parts, etc.?

If it's not going to a dealer, you're wasting both your time and the other guys as he can't fix that. Even if he replaces the part, the car then won't start until new software (dealer only) is installed so it'd have to be towed.

Search on "performance reduced" in the search field (below, to the
right) to see several threads on this. A quick glance shows it is
typically related to problems with the electronic controlled throttle.
(It appears somewhat common -- surprised a Volvo mechanic did not
know of it.)