How to prepare for a programming exam?

I study informatics and as you know we do many exams in front of a computer writing code for a given problem. Of course not all exams are writing programs, and some are even on paper (like maths). Now the thing is that very often i go super prepared for a programing exam and still get stuck while debugging the program or solving a simple problem for a long time. At home, i solve last year tests in half the time that is given for the exam and yet when i go to college to take the exam i struggle to solve this years. I am 3rd year and so far, when i take a non-programming exam i usually get what i expect depending on how much i have studied. But with programming you can never know. Even if you are super prepared you can still get stuck with something that doesn't depend on the material that you have studied for the particular exam. Certainly, writing code fast and efficiently comes with experience and practice but can you give me some tips on how to prepare in order to stay calm and focused on a programming exam.