So, the engine is «fighting» this added resistance during the entire intake stroke, which causes the «pumping losses» that are always mentioned. (greencarcongress.com)
The valves can open several times per intake stroke, based on engine load. (en.wikipedia.org)
You can have different quantities of air entering the cylinder for the same intake stroke duration (30 ms in your example), depending on intake manifold pressure. (mechanics.stackexchange.com)