Those activities that were popular in the past are no longer filling up. At first he thought it was just a temporary dip, but the trend is downward and has been for some time. This is not just a slow time; there is something fundamentally wrong with the program.
Slow, as far as I know means "not moving, acting, or happening fast" or "happening with delay" or "not clever" when referring to a slow learner. So, generally speaking, it is about low rate of something moving or progressing. However, I cannot understand the meaning slow time in the above paragraph.