My immediate thought was are you measuring throughput or latency?
The latency of reading from disk is indeed very slow compared to CPU instructions.
A 3ghz clock speed processor is running 3 billion (3,000,000,000 cycles a second) and some instructions take 1 cycle. You get 3 cycles per nanosecond. A SSD or spinning disk access costs many multiples of cycles.
Read 1 MB sequentially from SSD* 1,000,000
That's a lot of time that could be spent doing additions or looping.