The speed does come with practice - once you've done a particular type of question a few times, you should hopefully begin to recognize them when they come up and you'll know how to tackle it without spending a lot of time thinking about it.
The only formula for percentages I remember using a fair bit in practice, actually more like a shortcut, is when finding a percentage increase. If for example you are told that a value increases by 20%, and you need to calculate the new value, the full step by step way to do it is to find 20% of the original value using 20 / 100 * original value, and then add that result to the original value to get the new value. But a much quicker way is simply to do original value * 1.2. Or if the value increases by 35%, then the new value is original value * 1.35, and so on. That definitely saved me a few seconds. I vaguely remember practicing those kind of questions backwards as well - so you'd be given original and new values and have to calculate the percentage increase or decrease ... but I honestly can't remember if any of those came up in the actual test.
In terms of skipping questions, then I'd personally say yes, if you are getting bogged down on a question and it's just taking too long, or if you really haven't got a clue, then have a guess and move onto the next one rather than wasting valuable time. But keep in mind that you can't return to any questions you've skipped, so it's really a last resort. If you do have a guess, as it's multiple choice, at least consider if there's one or more answers you can rule out, to increase your chances of guessing right. And if you find yourself in the position that you have several questions left to go and your time is almost up, then definitely use the remaining seconds to guess the answers to the remaining questions.
One thing to keep in mind, and from the comments I've read here I think others agree with me, is that the actual test questions are slightly easier than a lot of the practice questions. As I mentioned already, it's not so much the maths that's hard it's the time pressure - but I guess that's what we are being tested on, the ability to work under pressure!