It sounds like you are already performing very well academically, so kudos for that hard work! It is important to note that testing is an artificial environment, and it must be timed in order to be conducive to a full school day. In reality, you wouldn't usually need to be rushed when working out mathematical problems, or addressing a chemistry problem, etc. (of course, there are exceptions). 

Knowing this, there is absolutely nothing abnormal about taking a little more time than average to solve problems. The only solution to resolving slower problem solving (again, I use the word "resolve" but there is nothing wrong here) is to practice frequently. After enough practice, problems that commonly appear on tests will become algorithmic and you will be able to solve them quickly. Consistent and continual practice is your best solution to improve speed. 

However, do not approach the concepts algorithmically from the start. Make sure to always build an intuition for the material you are learning, or you will later struggle when attempting to piece together the various pieces of a discipline.