The is the example given in the video. Two design alternatives are used as design thought experiments.
Design 1 - Serial
Read images serially. Do a disk seek. Read a 256K image and then go on to the next image.
Performance: 30 seeks * 10 ms/seek + 30 * 256K / 30 MB /s = 560ms
Design 2 - Parallel
Issue reads in parallel.
Performance: 10 ms/seek + 256K read / 30 MB/s = 18ms
There will be variance from the disk reads, so the more likely time is 30-60ms