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