Hi, we also walked pretty slow in June & July of this year. Not exact, but I think the route from Tui to Pontevedra* is about 50% roads/cars. But in many instances the roads are through small towns and neighborhoods so it's not the same as walking along a major avenue or highway.
There are many opportunities to take "complementario" routes that may add kilometers but also add beauty. For example, we heard that walking in to Porriño on the official route is quite industrial but there's a clearly marked diversion that takes you along the river through trees.
More details in our Live from the Camino post, starting at day 10. Thread 'Slow and Steady'
https://www.caminodesantiago.me/community/threads/slow-and-steady.87626/
*We veered off the Central Route at Pontevedra to walk the Variante Espiritual.