Does it always take so long?
No. Narrating the thought process really slowed down the proofreading. Also, this is a heavier markup than what you’d hope to see at the proofreading stage, there are processes that are so rushed that you’ll encounter this many problems.
However! Proofreading is much slower than reading. While a typical adult might read 10,980 words per hour, a typical pace for proofreading is just over 2000 words per hour. (This is often expressed in pages but the number of words on printed pages varies wildly.) The pace can slow or speed up, depending on the following.
Faster if: few errors, simple format, nontechnical or familiar content, clean copy, no style queries required
Slower if: many errors, complex format, copy marked up, style checks required (and more if unfamiliar), contains equations, symbols, or foreign characters/symbols