The first stages of the Ingles benefits from being along the coast, and therefore, if it is hot in Spain, the coastal breeze should hopefully make it cooler. After Betanzos, about 40% of the walk is on forested roads, again, giving some chance for shade. The rest is much on roads in open countryside, where, if it is really hot, you would be wise to carry an umbrella.
that said, the benefit of the Ingles is that it is all in Galicia, which should be cooler than many other parts of Spain in the summer. It could also mean that it is wet.
the Ingles is not crowded compared to other Caminos, but it is getting more popular. the albergues are not large in terms of number of beds, nor frequent. You may want to think about booking some private accommodation, as an alternative to sleeping in albergues.
If you are only able to walk about 15km per day, then you may want to think about the following stages:
Day 1 - Ferrol to Xubia / Neda
Day 2 - Xubia / Neda to Pontedeume
Day 3 - Pontedeume to Mino
Day 4 - Mino to Betanzos
Day 5 - Betanzos to Presedo
Day 6 - Presedo to Hospital de Bruma / Buscas
Day 7 - Hospital de Bruma / Buscas to Sigueiro
Day 8 - Sigueiro to Santiago.
Consider also thinking about having your bag transported each day by Correos and walking with a day pack. This should help your walking and may enable you to walk further each day, reducing the number of days walk, if 8 days is too large a budget.