Most of the other answers doesn't take into account the update OP gave in their post, so I thought I'd throw in my 5c:

The problem: I cannot work normally when I know I will be interrupted any minute. How to continue working normally? Are there any tricks to concentrate and ignore this situation?

That problem is unsolvable. You are a human being. By definition, if you are aware that you can be interrupted with high-priority stuff any minute, you won't be able to work normally. Being aware of a possible interruption raises your adrenaline and/or vasopressin levels, depending on the exact setting.

You will be able to work normally as soon as you forget about the possible interruption, by doing something intellectually or emotionally engaging - but you will still be extremely annoyed by it when the interruption finally happens, and won't be able to work normally for some time after that, due to a sudden drop in adrenaline and vasopressin but also highly evelated cortisol level.

As long as you're a sentient human being and are not using hospital-grade drugs blocking the secretion and/or uptake/reuptake of hormones, there is no real trick here. You're being put in a stressful situation, and you're showing a normal reaction to stress.

This is actually a sign of both mental health and emotional sensitivity. If you really want, you can try to fight with it. I, personally, wouldn't. No job is worth getting numb and empty inside, IMVHO.

How to (...) not waste time waiting?

How to be productive while waiting for meetings to start?

This meeting was about our team performance and our salary raises depending on that. I couldn't stay careless as I kept repeating in my mind what I would say about my projects.

If you really want/need to be productive then, do things that are extremely low-priority and don't require any intellectual or emotional immersion (like mentioned by other people). I would look at other people in my team do, though - if they are slacking off while waiting, I'd also find e.g. a good online game to play or a nice video stream to watch :)

The real problem lies elsewhere, I think.

I talked with my managers, they don't get me seriously and say "Don't think about it, just continue working". They are not willing to change this process, while the only thing I want is to be informed what's going on (Do I have to wait 5 mins or 1 hour for example)

What you did was professional. What they did was unprofessional. There's no professional way to deal with people who behave unprofessionally, by definition, other than either not giving a flying -d-uck, or switching to another workplace. If a meeting (...) about our team performance and our salary raises is treated like that, it's basically like the top brass is saying "we don't give a flying -d-uck about your salaries, your time, your well-being or you at all" in your face - either you allow it for some reason and ignore it completely - or you don't, and look for other place to work.

Being treated like that should light a red light saying STOP in your head.

Also, concerning something I read in one comment here:

Do you plan to work longer at such a company, or is that a way to get fired?

Actually, that's a good point - if you want to continue work in a place when you will be treated like garbage, for any reason (e.g. a very urgent need for money, not being able to switch etc.) raising any concerns is not a good idea.

A meeting with your bosses boss is not when it's "time", it's when the bosses boss has time.

chuckle Nope. A meeting with my bosses boss is when we both have time. I'm not a slave - I'm a valued specialist in a highly competitive market. If my bosses' boss would treat me like OP was treated repeatedly, he would get my notice, as simple as that - and I would essentially expect the same from him. I'm not doing him a favour by working for him, and he's not doing any particular favour for me. We're exchanging my work for his money, and I don't recall anything about "slavery" or "obligatory loss of self-respect" in any contract I had signed :) If he wants me to wait - so be it, I'm still getting paid. If, however, he do[es]n't get me seriously when the subject is our team performance and our salary raises depending on that, then it's high time to update my CV again and browse some nice brand new job offers :)