Good question.
Generally we advise students not to read the whole passage (waste of time); you want to get the general idea of the passage initially. Reading the first two sentences from each paragraph is usually sufficient to get an initial feel and to actually answer most general idea, structure, and tone/style questions. Take notes on what you read. You are simply mapping out over here and do not get caught in details.. Obviously sometimes you may need to read a bit more, so for example if you see the third sentence starts with an opposition conjunction, read this as well (ALWAYS pay attention to opposition conjunctions). Or if you see that it is an extra long paragraph or a one paragraph text then you may want to look at the third and last sentences too.
The point is do not read more than you need to. Too easy to fall into traps and waste time- taking notes actually ends up saving time for most people (in case you are wondering).
As for specific quesitons, the mapping can help you locate some of those specific questions. Generally you should not ready more than 2 or 3 sentences per each detail question.
In terms of time, of course doing an initial reading may take you a bit more than a couple of minutes but it will shave off your time when answering certain quesitons. So your times seem good for now. The more you practice technique and not reading everything the faster you will become. It is normal for the RC to take a bit longer. Doing the initial work can go a long way to making you precise and efficient.