How do you best manage your time on standardized exams like the SAT?
On the reading sections, definitely read the questions first. I still think it's important to read the entire passage also, but knowing what you are looking for before you start helps immmensely! As far as math goes, do everything that you know how to do first, and on the ones that you don't make a quick educated guess (on the multiple choice), bubble it in, put a star by the number in your test booklet, and come back to it later! It's always better to get all of the way through the questions first and then spend any extra time you do have on the questions you aren't so sure about.
How do I write an impactful conclusion paragraph?
I could tell you the general "reflect-and-restate your thesis" and "don't add any new information that you haven't previously stated", but you've heard that all before. The most important thing is centralizing your main idea that you have been trying to convey throughout the entire speech or paper, and bringing it home in your conclusion. Write one powerful sentence that would be the one thing you want your readers to remember if they could only remember one, and then make it personal to them. I like to end my conclusions with either powerful declarations of we as people, a nation, etc. or with a challenge or question to my audience.
What are the best ways to give a speech well and authentically by memory?
I believe that simplicity is always the best idea. If you are going to lose yourself somewhere and forget what you are saying or get tripped up over your own words, it is going to be during your longest or most complicated sentence almost every time. Speak concisely and in your own words. I often like to write my speeches, memorize them, and then quote them while having a friend follow along on my manuscript marking anything that I am saying differently than I have it written. I then change my manuscript to match what I actually say, because that is more natural and doesn't sound forced.