Kurzweil built upon the Singularity ideas of Vernor Vinge. In nearly two decades, computers have continued to improve, algorithms have improved and AI has improved. The Deep Learning and Reinforcement learning approaches to AI are very financially successful and have made progress on super human vision systems and the complex game of GO.
Ramaz Naam observed that the intelligence explosion arguments assumed that an AI would be able iterate on creating superior AI. However, if better intelligence becomes more difficult then it limits the rate of improvement.
There are problems that become exponentially more difficult as their size increases. This is for problems like the Traveling Salesman problem.