Since the dataset is small enough so that it fits into the Linux kernel page cache, and since the benchmark is repeated for 5 consecutive times, first iteration

Since the dataset is small enough so that it fits into the Linux kernel page cache, and since the benchmark is repeated for 5 consecutive times, first iteration of the benchmark will be bottlenecked by the disk I/O but the remaining 4 will not - e.g. all data will be in RAM (page-cache).