First code review at my new job: senior rewrote my whole PR

300 lines, opened on a Tuesday. The reviewer's comment was basically 'this wants to be three functions and a test,' and then a second pass came with a rewritten parser of about 40 lines. I felt caught out for a day. Went for coffee with him and asked how he reads a diff so fast. He said he reads the tests first, then the code, and only then the diff itself. I tried it on the next PR and it's already quicker to spot what matters.