In جَاءَ زَيْدٌ يُسْرِعُ, the accompanying clause begins directly with an affirmative imperfect. Its understood subject refers to Zayd. This is a verbal circumstance, normally without circumstantial wāw.
In جَاءَ زَيْدٌ وَهُوَ يُسْرِعُ, huwa is a separate nominal subject and the imperfect functions within its predicate. Both may convey Zayd's haste during arrival, but the clause structure and its linkage are different. Consequently, the rulings about wāw should not be transferred mechanically between them.