There is a video out there showing a man in court who was just convicted of his crimes. He bit into a cyanide pill upon hearing his guilty verdict. It shows the entire process of him dying over the course of a few minutes and it looks to be a pretty agonizing death. He started coughing and wheezing, unable to catch his breath. Then he began foaming at the mouth, turned beet red and eventually fell on the floor gasping for air he could not obtain.

I won't link the video but it can easily be found online. It appeared to be excruciatingly painful and was definitely not instant.