A neophyte white belt's first test is usually the most difficult. Many times, such individuals have never been in the glare of the spotlight, and sometimes, it can be quite intimidating, seeing your sensei staring at you from the judging panel.
For that matter, even non-neophyte white belts (those who have experience in another system) can be a bit jittery. Back when I started training in Shuri Ryu Karate, I already had significant amounts of experience in Shotokan Karate and Tae Kwon Do. Despite all of that experience, though, my old Shuri Ryu teacher's testing always included a physical portion, where you run 1-2 miles (no problem), and then everyone has to do 500 front kicks in a row, combined with 300 situps / stomach crunches.
The 500 front kicks is what seemed like an enormous mountain. For the first 100, I was thinking that this was quite a workout. The next 100 were generating a bit of a burning sensation in the legs. The next 100 made it a fiery burning sensation. The next 100 made the legs go numb, and the final 100 were the most difficult front kicks I've ever thrown in my life.
Then came the situps...
After the physical fitness portion was finished, I was wondering how the heck am I going to be able to perform Kata Wansu, along with the Taikyoku series, and complete the ippon and kihon kumite sections, with what I thought were my last reserves of energy.
As it turned out, that was probably the most relaxed I had ever felt in a long time, during a testing situation, and the testing went quite smoothly.
Still, there was a moment in time during the kicks and the situps, where I was wondering how the heck are people supposed to do this... I am quite thankful for everyone else taking the physical portion, since having your classmates and comrades there inspires you. For that matter, we would even have people who weren't testing, take the physical portion to spur on their fellow classmates.
Each test after that wasn't nearly as difficult, once those mental barriers were broken, which is why I still believe that my shichi kyu exam was one of the most difficult I had ever taken.