We present measurements of the leader fraction of neutron monitor counts that did not follow other counts in the same counter tube from the same cosmic ray shower. We use time-delay histograms collected at the Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand, which has the world's highest vertical cutoff rigidity for a fixed station (16.8 GV). Changes in the leader fraction are a precise indicator of cosmic ray spectral variations above the cutoff. Our data set from 2007 to 2017 spans a full cycle of solar modulation, including the all-time cosmic ray maximum of 2009 and minimum near the end of 2014, the count rate now having returned to its initial value. The electronics to collect time-delay histograms have been upgraded twice, and we have corrected for such changes to develop a long-term leader fraction dataset. We examine the spectral variation of Galactic cosmic rays above $\sim$17 GV resulting from solar modulation and its solar magnetic polarity dependence.