A few quick points - I notice you've posted a lot of questions in a very short space of time, ranging from performance tuning to example functions to more specific questions. Whilst this is what the forum is here for, you may find that a lot of your questions have already been answered - try using the search function as part of the forum here, or try searching up 'AskQV' which is a good site for being able to search for relevant articles!
Also, as many of them seem like interview questions (or similar ideas), it may be worth checking out some of the 'new to qlikview' materials floating about, there are a number of good books you can read to help you, as well as plenty of free resources (check out the training part of the Qlik.com site to start with!).
Also, just remember to check where you're posting your questions. You've posted this about QlikView in a Qlik Sense forum, meaning it's unlikely to get the best help - try the 'New to Qlikview' forum!
For performance tuning though, check out Re: Document Analyzer V2 -- a major update from Rob Wunderlich, which is a great app to help identify and improve performance issues.