You already asked a question about your categories problem (with little to no information that would allow anyone to debug the issue). I posted a comment there.
The rest of your questions are the opposite of "specific problem." Does something about your setup not work (other than the categories problem)? If so: what is the exact problem? Please describe (in a new question) what you expect to happen, and what actually happens.
If your question is about installing Q2A: did you follow the installation instructions (https://docs.question2answer.org/install/)?
What is your operating system, webserver, and PHP version?
If you're experiencing performance problems: how do those manifest?