Complex sentence: Or, it may be that the mystery user at the keyboard and John both type at 50 words per minute; but John never really learned the numbers, and always has to slow down an extra half-second whenever a number has to be entered.
Split into simpler sentences: Or, it may be that the mystery user at the keyboard and John both type at 50 words per minute. However, perhaps John never really learned the numbers, and always had to slow down an extra half-second whenever a number had to be entered.