If you are left-handed—as I am—you may never realize that you are writing against gravity and the paper. Writing with your right-hand enables you to let gravity help as the pen slides across the paper, but the angle of a pen pushes against the paper and creates extra friction that impedes the flow of ink.
I only recently learned this on a podcast, which also recommended a space pen to solve this problem. The ink is in a pressurized cartridge inside the pen and gravity is not needed to help the ink flow. I could write upside down if I needed to, but so far I am still just seeing if it makes a difference flat on my desk.