I was wondering whether If I can comment on their system as "slow" and
tell them that I have used faster libraries/APIs/technologies, which
are faster and they might use them?
Tread carefully here.
Make sure that worrying about performance is something relevant to the role you would be performing.
If your role in this company would be as a Performance Engineer and you would specifically be tasked with speeding up the system, you might try to see if the interviewer is open to discussing your observations. "While preparing for this interview, I took a look at your system. Would you like to talk about what I see?"
Then watch the reaction carefully, and try to gauge how receptive the interview might be. For example, you might be talking to someone who has no knowledge of these libraries, and you would be wasting your time. Or this interviewer might be very interested in your findings. Let her/his reactions be your guide as to how deep you want to go.
If I can ask, what sort of attitude and behaviour should I have? If I
cannot, can I ask something similar that would give the interviewer a
positive thought about me?
Again, be careful here.
Don't accuse. Don't make it sound like the current people are stupid for using the current technologies. The interviewer might be the one who decided that the current technologies were appropriate.
Remember, there are many reasons for using certain libraries/APIs/technologies - performance is only one of the reasons. Ease of implementation, licensing, maintainability, etc - many other factors may have gone into the decision; factors into which you don't have insight. Speed may be only a minor factor in the bigger scheme.
And of course, the company may be too invested in these systems to change at this point, so criticism won't be constructive.
You don't want to come across as someone who will complain about all past decisions. You don't want to come across as a one-trick pony ("I know technology B, so I'm going to recommend it every time I can.")
If the interviewer asks "So what do you think of our technology here?" it's an invitation for careful constructive criticism. But if not, use the tone of the interview to sense if your observations will be well received or not.